You're free to adhere to the progressive view that all people are essentially the same and interchangeable, but do you not even understand the view point of those who disagree? It's pretty simple, really; people from different cultures have different viewpoints. They don't all agree with your progressive liberal "anything goes" worldview.
"Worldview"? Until the rise of the nation-state post-Westphalia, empires and monarchies did have citizenship rules and rights based on place of birth, service to the empire/crown, and such. Yet twenty-five years in the military, regardless of your race/color/background, got you Roman citizenship, no ethnic preconditions. The Hungarians are about the only ones famous for requiring learning the language to become a citizen, an effective barrier. Cf. Hungary today.
But in the past, loyalty to a ruler was the common essential element, not pure ethnicity. In fact, empires prided themselves on containing many lands and cultures. No empire collapsed solely on the basis of not being ethnically pure. Meanwhile, local markets and trade routes were full of a broad mix of peoples. Silk Road, anyone? It's the spanking-new nation-state that has given rise to hard borders and which has promoted greater intermingling of those within to the exclusion of others, creating an illusion that living in ethnically pure communities is the historical norm, that all of history was the same as today.
Snowflake cultures afraid of "anyone not
exactly like me" are a right-wing fantasy designed to (1) provide a false,
unearned sense of superiority and (2) divide, conquer, and rule, as per Trump. Doing wonders for South Korea and Japan, btw, both in line to disappear.
The US is one of the very few first-world nations not facing a dire demographic meltdown, and that is very much thanks to messy immigration. Ditto Spain in Europe, doing better than many of its neighbors, particularly Italy. Yet so many fail, utterly fail, to accept non-Aryan immigration at the very same time they are happy to order pizza or enjoy rice pudding (from Africa, not China), unaware of their patent foolishness and the very real fact that they, too, are multicultural themselves.
Nice try. Whining is not an argument.