I think this might have contributed, but a larger contributor is likely the explosion of housing developments with larger lot sizes and suburbs-of-suburbs/exurbs, which prevent a kid from knowing any other kids living within a mile radius.

We're a part of the problem, my daughter is lucky she can walk to one of her schoolfriends houses. Her other besties are a minimum 30 minute walk, but they all attend the same school they're zoned for.