1) If you don’t have kids (socializing with other parents).
2) Don’t go to church/synagogue/etc (church attendance in decline)
3) Do not live where you grew up. (People chase opportunities in other cities.)
4) Aren’t in school any more.
Neighbor friends are also possible, but it can take many years for people to become comfortable enough to be friends, and due to #3, there’s often not enough time for that to happen before people move.
Half my friends are from work and half are parents. They’re not super shallow friendships but also not soul-level deep, either. Deep friendships were left behind from college days.