Lack of bonding experiences, especially facing adversity together, like when serving in the military.
There’s a lot to be said for being in a Fraternity/Sorority when in college, those are the kinds of things that create lifelong friendships.
I stayed with my teenage grand-girls while their parents went on vacation. It wasn’t until the week was half way through I learned THEY were keeping tabs on me at any given minute via their cell texting to each other!