Apart from having less time, I don't think it actually is that hard to make friends.

I think it's much more about this self-selecting you refer to.

It's like sampling food. If I go to a new country, I'll usually try everything once, but eventually I discover things I like and some things I don't like or aren't worth the trouble or the gut problems.

Why would I continue to eat food I know I don't like?

I've put some extended thoughts here: https://medium.com/the-taoist-online/is-making-friends-as-an-adult-really-that-hard-9f44d86b3bf3