It's the same situation as Denzel Washington playing Macbeth recently, Lin Manuel Miranda playing Alexander Hamilton: film and stage productions aren't about "ethnographic accuracy," it's just not in the nature of the medium. Similarly with old plays having all male casts.I really dislike this sort of nasty implication. I'm talking about world-building, not race relations. I don't care about the skin color of the cast, just how it clearly ignores any pretense to well-considered world-building for the sake of pandering. From a world-building perspective, the ethnic diversity of the Two Rivers and the world in general doesn't make sense. It could make sense, but they haven't bothered to make it do so.
And just to be clear, it isn't a huge deal, just noticeable and one of the facts that points to sloppy world-building.
Plus, what you say just doesn't make sense. If they "got over their racism 3000 years before" then they'd be more homogenous.