It's incredible (and sad) how few people around argue in favor of the DMs, these days. No wonder people have trouble finding games and DMs to run them...
I speak from the perspective of someone who is perennially the GM for my group.
Both you and Oofta, below, seem to be speaking as if the GM is a position that deserves some entitlements. You seem to be forgetting that your game is in competition with
every other entertainment, an embarrassment of riches, your players have at their disposal. Your players, by and large, are not
itching to play your game like they may have been decades ago. If they don't play your game, they're going to go off and watch the next season of The Witcher, and then maybe play some Elder Scrolls. Or maybe they'll get some more time at home with the kids, or get some laundry done. It may not be the same as TTRPG play, but... they'll be okay without you.
Yeah, maybe they have trouble finding games and DMs, but... they don't actually need them as much, either.
What harm does it do to accept that the DM has a vision of the world that doesn't include birdmen?
Your vision is not a thing they care about in and of itself. You need to
sell them that vision, and make it seem worthwhile and interesting
for them. It isn't sufficient that
you will feel more satisfied - it has to satisfy
them, because they are the ones who are actually impacted by it.
Corollary: The restrictions you enact actually have to be a clear part of making it seem worthwhile and interesting for them.
So, for something like human-dominated game worlds - you have to ask yourself how much more interesting that actually is for the player. Answer: If you aren't
using the fact that it is human-dominated for something, if it isn't an
active element, then it won't be more interesting for the player. It is just a fact of life that, after character generation, they are likely to ignore.
A real life example - I played in a d20 Star Wars game some time ago, and the GM placed a restriction - for
all characters, their first or second level
must be in the Jedi class. After that, you could do what you wanted. This restriction was an active plot element - our game was set after an alternate Jedi Civil War, in which the Jedi were outlawed in the Republic, and largely eradicated. It was central to the game's plotline that we were all Jedi.
What's central to your game that the PCs are almost all human? What will they get out of your restriction that they wouldn't get otherwise? A vague, often ignored sense of "realism" is unlikely to seem much of a payoff, to them.