This really isn't just an American thing. There are plenty of xenophobic European countries, and Japan and China are awful this way as well.

Interestingly, it’s mostly those who have never seen the people they’d have to interact with – once they have, they usually notice they’re the same as all other humans.

For the most part that's true but any tiny difference is used against a group if it fits in some other group's argument. It's funny how Asian, Hispanic and African descendant people in the US are singled out while all the Irish, Italian and Jewish are only viewed separately on second inspection. I mean, if it's all white-looking people in a place, they differentiate Italians, Jews, Irish, etc., but before that it's the skin color (on level 1). I suppose this is some natural reaction homo sapiens has had for millions of years and not necessarily ill-meaning discrimination. Some anthropologist with more insight here?

Yes, and given their cultural background and looks they mixed in more quickly than other branches. Anytime I discuss this, I'm amazed by the way Britain has integrated their African population and to some extent Indian and Pakistani as well. Though, I'm not an expert on the British mainland demographics and culture and I may miss the finer points.

Whites in the south interact with African Americans all the time tho.

Citation?

It's not a phenomenon unique to America.

It might even be... human nature.

We're in-group/out-group animals.

Absolutely. I believe it is completely human nature. No matter how advanced we may feel that we are, in reality we are all very tribal.