I think a big part of it is that adults are busy and have substantially less time and energy to just do things for fun or socialization.
I have always had a lot of time for myself. I am fortunate in that regard. Even now, I have the luxury to choose how to spend my time each day, and yet, I have no friendds. It's beginning to scare me.
As an older individual who moves around a lot, this question has increasling become important to me. Perhaps some of you are in a similar situation. While I didn't find the article particularly groundbreaking, it was a good reminder of simple things to facilitate the friendship making process.
Find friends through hobbies. Go practice a martial art, join an adult league of whatever sport you like. The best part is anyone you meet already enjoys doing the thing you want to do in your spare time, no need to convince them.