Speaking of fantasy worlds, serious question. Other than books specifically set in FR or other D&D property, how many fantasy novels have anything similar to the number of different races that D&D has?
I mentioned the SpellSinger series from Foster, it was populated with all sorts of anthropomorphic animals. But most fantasy novels I've read? Only a handful. Then again I freely admit my selection is somewhat limited.
Oz, maybe? Though in that case we're talking more about a variety of unique (and at times borderline freakish) entities and one-offs than "races"…
I don't consider "Because it's my world" or " Because I'm the GM" as satisfactory or meaningful answers to the question.
Not everybody is DMing for a coequal group of friends in a more or less egalitarian social situation. The dynamic can be utterly different in, for example, a long-running open table where most players who join the game are (a) joining after the campaign has begun and (b) are joining the game in response to posted advertisements or the campaign's word-of-mouth reputation. If somebody I don't know rolls up to my table and asks to join my game, great—but if they
demand to play a race that doesn't exist in my setting because it's in Volo's or Tasha's or Van Richten's, I mean, that's just rude
for starters.
Now if they
ask politely, "can I play a Tabaxi?", my response (based on the setting I'm running now) will be along the lines of, "This is a setting with a very intentional Middle-Earth vibe. Do you
absolutely need to play a cat-person, or are you okay with the standard lineup of humans, elves, dwarves, orcs, and hobbits?" And if they're very set on Tabaxi, the next question is going to be, "Okay, we can have your character be from some very far away corner of the world that isn't well-known, but there aren't exactly mechanics for Tabaxi in this edition—are you cool with playing a Rakasta, and with being the
only Rakasta around for a few thousand miles, and maybe the first and only Rakasta every NPC you'll meet has ever seen?" And if yes… "Okay, roll 3d6 in order. You need to roll Strength 9+ to qualify for the Rakasta class; good luck!"
(And based on the setting I was running the
last time we all had this argument? The answer from the get-go would be: "
Hard no. There are no cat people in this world. Why? Because I'm the DM,
and I made this world up, that's why!" The player is free to vote with their feet if they don't like it; it's no skin off my nose, I've got plenty of players already and more all the time. It's very much a referee's market these days.)