I think the opportunities to make friends in the adult world are drastically harder than as a kid with school.

With school you’re basically forced to interact with people in your age group, and region to become friends with.

Outside of school you’re not forced to go anywhere, so there’s no commitment to make any long term relationships since you can avoid people if you want to.

I’m not sure why this is the case but I think it’s because a fair portion of adults are burdened with families by that point, and make friends with the parents at Baseball, soccer, or American football practice groups, or whatever.