Hussar
Legend
Personally, I blame Tolkien for a lot of this. 
Before Tolkien, with writers like Howard and the like, the fantasy world really was mostly like our world, just with a smattering of magic. The world's population was human (mostly) and fantastic races were mostly limited to a single location. There was no "fantasy races". ((Yes, I know I'm painting with a wide brush here))
But, then comes Tolkien with Middle Earth - this fantasy world that makes absolutely ZERO sense but everyone loves where you have dwarves and elves and goblins all with their own nation states and cultures and whatnot (that are mostly just human cultures grafted onto races). Almost no thought given into how having living immortals (elves) would absolutely rework everyone's cultures - didn't matter. We get the faux-medieval setting and, so long as you don't start picking at threads, it works for the story.
And, then, after that, everyone and their mother writes their fantasy world like this - Moorcock's Melnibone, Donaldson's The Land, Brook's Shanarra - all built on this same model where the implications of the setting are completely ignored in order to get this ren-faire medieval setting.
Now you see all this pushback against it too. The whole "Cantina scene" criticism of D&D worlds. But, the Cantina scene makes just as much sense in a fantasy world populated by dozens if not hundreds of intelligent, sentient beings as a world where apparently everyone believes in Apartheid. People just don't like the fact that it's just as plausible that you have races freely mixing as having a setting where everyone wants to keep all those "other types" out of their communities.

Before Tolkien, with writers like Howard and the like, the fantasy world really was mostly like our world, just with a smattering of magic. The world's population was human (mostly) and fantastic races were mostly limited to a single location. There was no "fantasy races". ((Yes, I know I'm painting with a wide brush here))
But, then comes Tolkien with Middle Earth - this fantasy world that makes absolutely ZERO sense but everyone loves where you have dwarves and elves and goblins all with their own nation states and cultures and whatnot (that are mostly just human cultures grafted onto races). Almost no thought given into how having living immortals (elves) would absolutely rework everyone's cultures - didn't matter. We get the faux-medieval setting and, so long as you don't start picking at threads, it works for the story.
And, then, after that, everyone and their mother writes their fantasy world like this - Moorcock's Melnibone, Donaldson's The Land, Brook's Shanarra - all built on this same model where the implications of the setting are completely ignored in order to get this ren-faire medieval setting.
Now you see all this pushback against it too. The whole "Cantina scene" criticism of D&D worlds. But, the Cantina scene makes just as much sense in a fantasy world populated by dozens if not hundreds of intelligent, sentient beings as a world where apparently everyone believes in Apartheid. People just don't like the fact that it's just as plausible that you have races freely mixing as having a setting where everyone wants to keep all those "other types" out of their communities.