Part of that stems from our immigration services actively directing immigrant influxes in ways to spread them out and discourage them all settling in one place, forming a closed enclave. Humans naturally want to be among those who understand them and share history and culture. But, that can also hamper assimilation.

Of course, that hasn't stopped those enclaves from later developing as immigrants open businesses catering to their own. It seems to draw them together again rather organically – as with Iranian immigrants in the Washington DC area, for example.