Came here to say what you already said: the built environment accounts for a great deal of this. It's simply not possible in a great many parts of Middle America to walk or bike over to a friend's house, and to the extent any social fabric exists, it is built entirely atop parents' willingness to drive you over for coordinated and time-boxed play-dates.

I have been arguing for almost my entire life, as a European immigrant, that built environment and automobile sprawl shapes relationships and cohesion. I was constantly dismissed and told that these are superficial differences, that people are just as lonely in dense, transportation-rich urban jungles, and that motivated people in the right cultural context can defeat any environmental obstacles to friendship and connection.

I hope the tide on that is starting to turn. Built environment isn't everything, but it's a lot.