Sometimes they do. I think laziness is an entirely valid reason for a DM to impose restrictions on their game. Only running a game to a certain level comes to mind. My last campaign went from 0-20 and lasted 5 years. Running high-level games is a lot more work than low-level games (for me at least). I feel fully justified to state that I'm going to run a shorter, lower level campaign for no other reason than I don't want to put in the work.
I'm just as comfortable not allowing classes or spells that I'm unfamiliar with or am familiar with, but find more difficult to run games where they exist, for no other reason than I find it harder to do so and don't feel like putting in the effort to familiarize myself with them or work around them.
Actually, laziness is perhaps the best reason for me to disallow something in a game I'm running. I'm doing this for fun. If it starts feeling like work, I'm not doing it.
I wouldn't call that "laziness." To me, "laziness" as the reason to disallow something is more like...
"Eh, too much effort to go look up the rules, so I'm gonna say no."
"But...you own the book! It's right over there!"
"Yeah, I know. I just didn't feel like getting up to read it."
"I am not prepared to deal with the added DM responsibilities entailed by high-level characters" is rather a different beast--that is specifically setting out in advance that you do not
wish to do that work, before it is required of you, rather than shirking it on the spot.
So one thing that strikes me odd that some people simply seem to want to play some species in a vacuum. Like they don't know or care what the setting is like, they just want to be an elf or something. But certainly elves of different worlds are pretty significantly different things? Tolkien's elves are not Pratchett's evens and Athasian elves are nor Faerûnian elves and so forth. So even if we assumed that elves existed in the setting, I still would first want to know what elves of this world are like before I would decide playing one.
Sometimes they are. Sometimes they aren't. I find that the vast majority of campaigns that stick to the "core four" races
haven't bothered to make them in any meaningful way different from "Tolkien-esque with the serial numbers thoroughly filed off." It's one (of several) reasons why I expect, as part of any pre-campaign discussion, that the DM actually...y'know, make a case, sell me on all this stuff they've put so much time into. And why the rather dogmatic "I'm the DM so what I say goes, like it or lump it" attitude leaves me so cold--that very specifically reflects a refusal to bother selling the player on stuff, and from what I've seen, a rather lethargic attitude toward having the stuff you'd need to make a sales pitch in the first place (beyond, y'know, "I wrote it," which...yeah that's not exactly a great sales pitch.)
I say "almost" because, in serving the utilitarian purpose of serving up both high fantasy/JRRT-esque tropes and pulp/S&S/REH-esque tropes, it does convey some of the same aesthetic as those works of fiction. But the fact that it does both at once in itself betrays its limits as a work of art - it has no real depth at all.
Which is fine - the depth can be brought in play; that's part of the point and pleasure of RPGing.
Indeed, that's precisely where I prefer to see it. That's one of the other reasons why I'm left pretty cold by someone declaring that they've nailed down every possible culture, race, political faction, etc. for the entire globe (or equivalent, for non-round worlds.) It communicates to me that I'm not supposed to find (or see) much depth through play--other than by witnessing the depth that's already there.
I don't fully understand though why people drop into these behaviours instead of choosing other routes. Is it a defense mechanism of some kind? Probably, but I couldn't say for sure.
As someone with social anxiety, I can promise you that it is a defense mechanism. Think of it this way: People you do not know are
threats. Unknown people will
hurt you. Hence, you must keep unknown people at arm's length, so they can't hurt you. But if you've forged a connection with someone, if you've been able to lower your guard enough to let them in, well. Now they're inside the castle. Letting someone in, only for them to then do something upsetting, feels like the same hurt that caused all this fear and anxiety to begin with. Hence, to "betray" friendship is a pretty horrible offense. Likewise, if
you trust person A to be good to you, and you trust person B to be good to you, well, they
have to get along, right? You know they're people who don't hurt others, therefore they can't hurt one another.
Almost all of this is rooted in maladaptive coping mechanisms, often as a result of ostracism and unresolved feelings of inadequacy, coupled with a particularly pointed desire for socialization (since the avoidant behavior leaves the person starved for social interaction and, in many cases, regular human affection, outside of family members.)
To sum up, in a non-geek/nerd group, someone bringing up the request I mentioned upthread would be met with first, consideration. If that request went against the grain of the group, it would be politely rejected. At that point, the requestor would either accept the fact that they were part of a social group and go with the group's decision, or take the initiative to find another group who more closely fit the parameters of their request. There would not be any sort of social trauma resulting from this. Some of the responses in this thread suggest a different result to that interaction, hence my link to the
Five Geek Social Fallacies.
As said, I don't personally think it's relevant. For me, this is not about identity, but rather, about being respectful to one another. A respectful person does not throw their weight around in this way, regardless of their position within a group. Many here have made a great deal of the extra "work" or "responsibility" of the DM, which I personally think is more than a little overblown (having been a DM for several years now). Further, I think such harping on that to the exclusion of other information is ignoring the fact that the DM is the one with most of the
power in this social dynamic. Thus it is incumbent upon them to use that power judiciously, with caution, restraint, and magnanimity. I see rather a dearth of all three in most descriptions of blanket bans,
particularly given the disinclination, or even outright hostility, to the very
idea of the player asking for a good-faith discussion with both sides trying to achieve consensus.
In the real world everyone at the table should be looking out for the enjoyment of everyone else. That means that if a player knows that the DM's fun would be negatively impacted by playing a dragonborn, it would be crappy for that player to arrive at the game the DM set up and try to play one. Similarly, if it doesn't really matter to the DM or setting for a warforged to exist, but the DM has said they don't exist, it would be crappy of the DM to deny the player his fun character.
I guess I just find it really hard to believe that "one person is playing a dragonborn" is such a horrific, onerous burden that it would "negatively impact" the DM's fun. That notion is genuinely baffling to me. They have an entire world--an entire
universe--at their fingertips. But one player playing a race they aren't into is now, apparently, enough to spoil the experience. That's just...I genuinely cannot think of any other way to describe that but "petty." It just feels
petty to have an experience irreparably damaged by something that
small in comparison to what the DM personally is engaged with. As if the mere presence of an avocado dish at a banquet were enough to ruin the guest of honor's appetite, because they find avocado greasy and mealy. (Which, for the record, I do!)
And, because I'm sure someone will make one, check your slippery slope arguments at the door.