California has been that way at least since I was growing up there in the mid-70's. Your friends are not your neighbors. Your friends live 20 miles away and you usually only see them at work or at occasional weekend events.
Try to import the Midwestern "block party" ethic and you will just get ignored. It's not their culture.
I live in a small NY town with a near 200 year history. Neighbors know each other. Some are quiet and don't want contact, but many others stop to chat, will stop by for a burger on July 4th, help each other during a big snowstorm, etc...
Have some siblings who moved to SoCal and Phoenix. their neighborhoods are strange to me. People are generally polite, but never mix, as you point out. Most are transients who seem to move every couple years, but even those who have lived side-by-side for 15 years never speak. Also being transient in nature with a lot more turnover, you can see fast changes (i.e. declines) to the neighborhood. Watch out when the first welfare queen and her fatherless son's pack of pit bulls move in....
I grew up in a Southern California suburb in the 60s and 70s and our neighbors all knew and socialized with each other.
We were told America would be "transformed" by the Kenyan occupant of his White Hut. Mission accomplished.
“Try to import the Midwestern “block party” ethic and you will just get ignored.”
Maybe, but he’s not even doing it right. The key to a midwestern “block party” is not baking bread, it’s tapping a keg of beer. That’s how you really get to know your neighbors better :)
Elites have spent many years ensuring you’d have little in common with people around you (even your family); they’ve divided people along ethnic, religious, class, gender lines - very effectively.
In my area (northeastern NJ) you probably don’t even speak the same language as your neighbors.