Add language and religion to the list of tribal affiliations and replace racial segregation with ethnic segregation, and you have the way the US sees the rest of the West.
It seems like we're all playing the same game, just with different pieces.
It's not that people don't mix, it's more that they define their identities by multiple tribal affiliations. And "tribal" doesn't just mean race, it means anything from Mac vs PC vs Linux, to frat houses, to financial traders, to progressive academics, to LARPers, to indie musicians, to YC applicants.
Plus the usual political camps.
There's a patchwork of scenes and monocultures which don't seem to have much to do with each other - which is a different and less permeable dynamic to just being interested in something, or working in some field.