Specifically white Americans want to avoid living with brown Americans. Once segregation was no longer law, it launched a wave of destruction for public infrastructure: pools, schools, transit, even things like sidewalks all became things to oppose if they had to be shared.
I don't know that it's uniquely American thing either, I think this happens anywhere that different cultures come together. Humans are deeply tribal.
Check out Shellings model of segregation [0]. Global seggregation doesn't necessarily mean that local decisions have strong preference for their race.
e.g. for two types it's enough to have preference "I want at least 33% of my neighbours to be same type as me" to reach segregated global result.
[0] https://en.wikipedia.org/wiki/Schelling%27s_model_of_segrega...
That’s definitely not perfect (and it’s like, say, Chinatowns weren’t raided or sacked) but that option of coexistence wasn’t on offer in much of the country.