TL;DR: it is because America actually has a high number of people of different races. Couldn't turn any different. Any sufficiently rich country (so people can

TL;DR: it is because America actually has a high number of people of different races. Couldn't turn any different. Any sufficiently rich country (so people can afford to choose where to live and in which kind of dwelling) where significant portion of people are of different race, will segregate. If formal segregation is banned, people will segregate using economic mechanism. And it still works this way because "first" generation of suburbs - closest to cities - have worn out, became affordable for nonwhites and increasingly populated by them so white flight continues further out, to more and more spread out sprawl.