Friendships seem to develop with people you spend a lot of time with, work being a big one of those, or have common interests/pursuits which bring you into regular contact. Of course these situations vary throughout life; you don't stay in the PTA group for your entire lifespan, though you might keep in touch with a few of them.

All I really know is it is easier to pursue friendships in retirement because people have more time for it. The modern middle-ager is locked into work, housework, yardwork, child care, extended family, maybe religion, and usually youth sports. Who can be surprised if their only new friends in adulthood are work friends?