Some gamers (DM's and players) view players as consumers. The DM produces a product, the players consume that product and play progresses. As consumers, the players are not expected to have a significant amount of input in what the producer produces, so long as the consumers are happy with what is produced. The players come to the session, play during that session and once the session ends, they don't play again until the next session and have very little interest in the production side of the equation.
Other gamers, myself among them, view everyone at the table as collaborators. Sure, the DM is likely going to do more work than any single player, fair enough. But, the players expect and are expected to contribute outside of the game. During play, sure, we play in character and whatnot. But, once play stops, the players put on their contribution hats and supply various elements - NPC's, possible connections, discussions about the future direction of the campaign, goals etc - to the campaign.
Does this make sense? If you're a "Players as consumers" type DM, then, player input isn't really sought or expected. You have your campaign, your world and the players can take it or leave it. OTOH, if you're a "Players as collaborators" type DM, not only do you welcome player input in the design of the campaign world, you expect it.
Actually, yes, this does make rather a lot of sense. It's not a perfect analogy, but it's good enough. When I DM, I solely produce the content ahead of time (which is to say, I craft the sandbox and fill it with contents), and when players play, the sandbox is in no way customized to fit those players or the characters they create. I can and do play the same campaign with different groups of players, and a huge part of why that's fun is to experience how it always turns out wildly different every time, even though I've started from essentially the same fixed point of origin for each group.
If you view gamers as consumers, as a DM you are going to want to produce the best product you can so that your consumers are happy. So, you create your world, create your campaign, and then the players play in it. The idea of creating the campaign or the world based on the characters the players bring (a la FATE systems) just isn't a consideration. The product is largely distinct from the players playing in it. The DM would run the same campaign whether it's this group of 5 players or that group of 5 players. And, since the players are happy, there's no problems.
Which mean, for the product model to work, you need to limit what the players can bring to the table. Again, everyone is happy, so, there's no negative meaning meant here.
The problem comes is when some of the group belong to the "gamers as consumers" side and others are "gamers as collaborators". It becomes a mismatch in expectations.
Precisely!
To be fair, the last player I had who made a cleric, not only didn't bother to name a deity, but couldn't actually name the setting we were playing in at the time.

And even after being reminded of the setting, couldn't name any of the gods of the setting. IOW, the player hadn't given the slightest thought to engaging the setting in any way, shape or form.
So, yeah, I am sympathetic to the DM's side here. I really am. It's so frustrating to take the time to develop the campaign and then have players swan in, not bother learning anything, and then give you grief when you happen to nix their latest creation because it doesn't fit in the setting.
However, all that being said, where I have a lot less sympathy for DM's is when the player HAS done his or her due dilligence and has come to the table with a workable concept, even though it wasn't on the menu that the DM laid out, and gets shot down. The samurai in the Egyptian campaign, for example - after all, in D&D timeframes, there is at least one documented African Samurai in the court at Edo in Japan. If I've come to the table with a samurai in Egypt that part of the trade delegation that settled here a few years ago, my character grew up here, has contacts within the setting and is tied to the setting, then I'm a lot less willing to side with the DM here. The player has an idea, he or she has done the work so that the DM doesn't have to, and is honestly adding to the campaign, then, yeah, at that point, as a DM, perhaps compromising a bit is in order.
In my experience, the vast majority of players simply want to play, they play casually, and they don't
want to be more involved than that. Maybe they watch YouTube, maybe they read the
Critical Role comics, but they don't post on EN World or know what the
Dungeoneer's Survival Guide is or get into flame wars on the internet about player agency and DM authority. Occasionally some few players get bitten the by the bug and decide to start buying sourcebooks so that they can approach their preferred game more in-depth as a player, or they want to take a crack at DMing themselves. But most are just happy to play their character(s) and play the game.
The bulk of players who are only involved in the hobby casually largely have no reason to "come up with a character concept" first and
then try to foist it on a campaign, potentially causing conflict. They come to the game with open minds, ask how to make a character appropriate to the game, and then they do that. And if that winds up not working out,
then there's discussion and compromise.
But now that I think about it, every time I've ever had a player try and create a character that wouldn't fit the campaign I was running, the player was anything
but casual, and the character idea
always seemed to come from a place that preceded the player joining the group. (Ever had a heavyset neckbeard insist on playing his Bayonetta expy in your Holmesian detective campaign? No? Lucky you.)
So again, no. I reject this idea that the only way to make a superior game is to subtract from the game. Great games can be had with wide options. Games with deep and compelling stories can be had this way. Stop insisting that your way is better.
Nobody here is insisting that curated games are the
only way to have good games. Great games
can be had with wide options. Great games can also be had with extremely limited options. Context, context, context (again). It depends on the game and the group. Surely you can't disagree with that?
You created a setting, handed it to the players, and they were inspired by something. Some connection they saw that you didn't see. I remember there was another poster a while back who said that there were no Forge Gods in their world.... because they hadn't thought of any. Does that make it bad that a player does think of it?
I mean, for a bunch of people who are so insistent that the written books are malleable to the point where a player should have no expectations that anything is in the game, you guys sure do like to make a list that is immutable, unchangeable, and constant. It is all guidelines, until you decide it isn't, and then it is adamant law
I don't see what's so hard to understand about it. Text is always mutable; that's the nature of text. It's subject to all manner of interpretation, emendation, addition, deletion, etc. Whether by the hand of a DM acting alone or a DM acting with the express consent and input of all players, the text can always be altered to suit the group, because it has no power over the people using it. Rules-as-written only have as much authority as the people implementing those rules choose to grant.
Rules-in-force, which is to say the actual game as it's played, are necessarily less mutable, because, well, the decisions have all been made. "
These are the written rules we're using,
those are the house-rules we're implementing,
this is how the DM makes rulings in corner-cases,
that is the set of precedents that inform rulings, etc." The rules-in-force cover everything from the finalization of those early decision-points (again, regardless of whether they're made by the DM alone or with player input) right up through the very present moment when the game is being played (under which circumstances it is nearly always the DM alone implementing and adjudicating rules, deciding "what happens" in the fiction as a response to player decisions, and for the most part acting
in those exact circumstances as the one inalienable "authority" over the game).
And this "play-in-the-moment" is actually the least mutable thing of all, because you obviously can't change what's happening
as it's happening without dissolving the whole game into a miasma of meaningless, chaotic discontinuity. Once a thing has happened in the game—once the DM has made a ruling—it
has to stand in order for the game to have anything like causality and an arrow of time.
Now, here's the rub: setting lore is no different from game rules. Call one fluff and the other crunch if you like, but there's really no difference between them for the purposes of this discussion. A published setting guide has no more authority over my table than a published rulebook. But
once I've made the decision—and again, you can say "I" meaning the DM or "we" meaning all the players here, and the argument doesn't change—it stands. It has to. If I've (we've) decided that I'm (we're) playing the 1983 edition of Basic D&D, but with a house-rule granting maximum hit points at 1st level, that's how my (our) game world works. If I (we) decide that I'm (we're) playing a setting with no elves and no tieflings, that's how my (our) game world works. Simple as that.
Next, understand that for some DMs,
creating the setting (and
writing mechanics and
implementing house rules) very much
is tantamount to finalizing those decisions that turn the mutable rules-as-written into the immutable rules-in-force. Hell, if the setting is persistent between campaigns, every aspect of creating the setting, deciding what happens in the campaign world between campaigns, advancing the fantasy world's timeline—
that's gameplay, as surely as the 0.5 seconds of present moment that it takes a DM to adjudicate what happens when a player thrusts a 10'-pole into the nozzle of an oil-trap.
And
that is how we go from changeable guidelines to adamant law. Who has the authority to make these final decisions will vary from group to group depending on the social contract, but for most groups by far, the longstanding traditional norm has been that this authority rests with the DM. It doesn't have to; it just often does.