I mean immigration existed, but it wasn't nearly as common as today. A lot of these IPs just plop a minority in an area where their presence would turn heads, have everyone act super casual about it because they are too lazy for a backstory, and then call everyone a bigot who points out this is sort of silly. On the flip side, there are people who will call creators bigots for not including minorities in some quasi historical setting, even if their presence was rare.

Like pretend someone was making a movie in present day central Africa. Everyone is central African. Except one dude who is pure blooded Navajo. No explanation is ever given, and the only people who seem to even notice his race is the villain.

While it's perfectly possible for someone of Navajo descent to find themselves in central Africa, it's not really that likely. Audiences would want an explanation, and would consider it unrealistic if absolutely nobody commented on it except some over the top villain.

There's also an aspect of gaslighting going on here. Over the past decade historians have made a lot of claims about racial compositions of historical groups that were later exposed to be largely inaccurate. While historical inaccuracies are always a thing, it's pretty convenient that all these inaccurate claims fit into the narrative pushed by American progressive identity politics.