An old rationalization of prejudice. Everyone seems homogenous to me and what was heterogeneous yesterday (e.g., Italians and Irish) is homogenous today. Just stop worrying about it. People with different backgrounds are much more interesting, all else being equal. All are Homo sapiens.

Also, kids in city neighborhoods also go to the same schools. In suburbs I've seen people don't generally share an employer and church - that's a small town.

It depends on the definition of suburb (some are pretty urban), but my experience in cul-de-sacs is neighbors rarely interact. Lots of places don't even have sidewalks.