My pet theory is the elimination of the third place[1] where socialization can happen. Much of this is because the push for higher and higher levels of productivity and profit has made people more and more busy, which leaves less time and energy for these third spaces.

[1]: https://en.wikipedia.org/wiki/Third_place

There are a lot "third place" spaces in New York and other big cities.

Would be interesting to see if New Yorkers have more friends/more social.