The Bowling Alone book captured this. https://en.wikipedia.org/wiki/Bowling_Alone? Personally, I subscribe to the idea the underlying cause is the loss of trust in American institutions (government, church, corporations, sports, unions etc.)

       - Decline in trust undermines institutions, and the loss of institutions further reduces trust.
       - Rising inequality, political polarization, and lack of fairness & opportunity further reduce trust.
       - This results in withdrawal from in-person activities and the substitution of online "life".