If your world exists separate from players and when you sit down with 5 other players, no aspects of it can bend-- you don't have a roleplaying game. I don't really know what you are doing, but ideally you should be open to altering the world the moment you sit down to play the game.
I'm going to use a brand new example setting, and I'd invite people to think about it.
Modern earth espionage game. Nothing supernatural, no aliens, etc.
...
Would
anyone here actually feel it is appropriate if you are
invited to play in such a game,
acknowledge you are on board with it, come into it with
a group of participants, to then ask if you could play a wizard, alien, etc?
If not, would you even feel that it is appropriate to be frustrated with the GM for inviting you to participate in such a restricted game?
I'm going to go out on a limb and say anyone who says "yes" is more likely to just be having fun debating rather than expressing a real opinion.
I submit that a DM-made setting has the right to its setting integrity in the same way as that setting. The only difference I can think of is that everyone understands the relevant details of the setting that I just described, whereas you may not know what all is entailed in the DM's setting. It could be somewhat annoying if you felt you were having a "mother may I" exchange with the DM trying to ask what you could play until you finally found something he/she would allow. The point of it is to make the setting clear enough up front that such back and forth is minimized.
On the other hand, if you have a world without a story, I guarantee it is not a particularly well-made world... which is precisely why you can't come up with an example.
See above.
Now, granted, I would be willing to concede that you can certainly make a world that is human-only should certainly be human-only, but that is part of conceptual theme. Once in the world someone can play something akin to Elves, it is just stubborn, bullheadedness that insists that means something akin to Orcs or Dwarfs or Catfolk or Dragonpeople or Wookie or whatever can't exist in the world nor be protagonists without disrupting the world.
Would it be stubborn bull-headedness to not allow magic in a hard sci-fi game? If you agree that the spy game's parameters above are reasonable, why wouldn't it apply to
any genre?
So if I am doing a table-top unscripted game, then you'd better have me hooked by being able to do something i couldn't do in a computer game and that certainly starts at character creation, but in general means valuing the creative input I and everyone else at the table contributes and incorporating it into the narrative.
I'm not sure why you think anyone is opposed to character creative input. Most world-building DMs are
even more likely to allow PCs to go off and do their own thing, making their mark on the world in whatever way they want to, than are DMs who weave their narrative around a story about the players. In that latter case, the social contract demands that certain assumptions about where the story canvas is going or about be adhered to. In the former case, the boundary lines are all drawn up before you create your character. Create any character that fits the world, and then that character can
do anything they want within that world. Topple kingdoms, become head of a trade consortium, sail around on a private yacht wherever their whims take them.
The question is whether character input is about world-
building or world-
affecting. World-affecting play is every bit as free and empowering as world-creating, but it is a very different experience.
SIDENOTE: It is psychologically a different experience to interact with something that you have not created than it is with something that you have.
And I probably wouldn't use D&D for it.
Because people come to D&D with certain expectations. They want the freedom to get excited about the possibilities the PHB has on offer. And I don't think they're wrong to want that. Unless you're all in with players who are specifically excited about some particular nonstandard campaign world, I think it's a little... much for the DM to shoot them down with "But, MAH VISHUN!" For the player who is interested in the implied D&D universe, it's fighting the protocols of the game. You can get away with doing so, but you really need to screen carefully beforehand.
This is an excellent thought to add into the mix. Is part of the objection based on specific
D&D assumptions? Because that would, at least, explain why I'm guessing a lot of people would feel it's okay in a spy game, but just aren't comfortable saying it's okay in a D&D game.
I think you're making it too hard OP. If you say "The setting is a grim and gritty post-apocalyptic, sci-fi wasteland" and the players make, I dunno... "magical anime schoolgirls" then you either failed to communicate the genre and tone you were going for, the players are idiots or the players TOTALLY get it and their characters are some sort of clever riff on your setting.
In any case, the solution for all three scenarios is the same: talk to them and listen to what they have to say. You might be surprised. If their reasoning isn't great, compliment the parts you think fit but ask them to incorporate those parts into a new character that fits the setting a bit better.
Even better, if you think you might be in a "magical anime schoolgirl" situation, ask them to run their concept by you before they roll it up.
I'm not actually having a problem in my own games, and the other DMs I know haven't had it either. I just see reactions online to the world-building DM, that feel to me like they are entirely misperceiving the situation. I've just seen too many times where people get it into their heads that certain types of DMs are stubborn jerks making it unfun for everyone but themselves (which would have to entail them being sadistic too apparently). In reality, those conceptions are just off the mark most of the time. My goal is to get people thinking and hopefully see past those misconceptions.
Now, like my sidenote above, world-building and world-affecting are different experiences. I've come to the point where I honestly feel an emptiness as a player if I have too much world-building participation. It feels a bit like playing Neverwinter Nights with your friends, with everyone in DM mode plopping down stuff with the creator left and right as you go along. Not really of interest to me at this point. Now, there have been times when I've found it interesting (the player world-building, not the crazy NWN scenario), but it is a different psychological experience--it hits a different spot. And the point I'm making is that spot isn't about being an unreasonable DM who wants to stop everyone else's fun, it's about focusing on a different type of fun that is more or less mutually exclusive with participation in world-building. Neither of them strike me as superior or inferior, and it's definitely fine to have preferences. I strongly dislike mushrooms. I have no use for them. At least once, I've tasted mushrooms that I could tell were exquisite. Quality was oozing out of them. Heck, the texture was almost appealing. In that situation, I could appreciate their quality quite clearly, and see how others would enjoy them. But, I still didn't like them because they tasted like mushrooms. So I feel like a lot of people are entirely failing to appreciate the quality of the experience of playing in a world-building DMs world. One may or may not like it, but it should be a matter of taste, not a false value judgement about the experiential quality.