So I went through the Monster Manual and asked "Does this exist?" you could answer yes or no for each and every monster? Because you can do so for the PC options I wager.
See, I don't believe many DMs actually care much about the internal consistency of their world. That's a front. If they did, they'd know what monsters (and spells and the like) exist in their worlds. Dragonlance, for example, is EXTREMELY specific about what doesn't exist (orcs, lycanthropes, etc) and I don't wager most DMs bother with that. Because they don't need to spell it out since it would be a list for one: the DM and if the DM decides xvarts are stupid, they don't need to say that they just don't use them ever. Instead, they spell out the PC options since that list is for more than just the DM and they must be careful in case someone actually says "Can I be a X?"
In short, I don't think artistic vision has much to do with it, I think it has a lot more to do with "rules for thee, not for me."