Being part of a Nuclear family doesn't mean that you can't branch out and be more involved in a community. Raising kids is hard, and can be lonely but also gives you a strong bond and shared experience to talk to a HUGE part of the world and make introductions.

The community, layout of housing, etc are all extremely impactful on this. So too, is the ability of the community to organize. You can have a GREAT community in a cookie cutter suburb. This is especially true if you have communal gathering areas like a pool, park, etc.

I guess what I'm saying is that the Nuclear family is not the underlying reason for any of this. The ability to be alone if one wants to be alone for all things (think about how technology has basically enabled you to function without any human interaction if you choose, and people DO choose this), and the idea that you're being rude by taking the risk of saying hello is a larger cause.

EDIT/ corollary: Advice to new parents - try to find a place with other young families. They're all looking for this connection. These are some of the strongest networks that you will find especially if they live in close proximity.