The simple answer is media. First television, then internet, then smartphones.
People used to actively seek the company of others - if for no other reason than out of boredom. Entertainment and parasocial relationships have filled that hole.
Another reason I might posit is declining birth rates and an aging population. Entire neighborhoods used to be filled with young families of roughly the same age. Now on any given city block in the US, there may only be 1 or 2 houses that have kids of any age in them. Smaller concentration of kids -> thinner market for playdates and sleepovers -> less interactions between families.
The simple answer is media. First television, then internet, then smartphones.
People used to actively seek the company of others - if for no other reason than out of boredom. Entertainment and parasocial relationships have filled that hole.
Another reason I might posit is declining birth rates and an aging population. Entire neighborhoods used to be filled with young families of roughly the same age. Now on any given city block in the US, there may only be 1 or 2 houses that have kids of any age in them. Smaller concentration of kids -> thinner market for playdates and sleepovers -> less interactions between families.