I think it’s mostly a time issue. Used to be much easier but with recession, inflation, housing crisis, constant layoffs at least I am much busier than I was e.g. 10 years ago.
The little time I have for going to the gym I kinda don’t feel like talking to people I’ll probably never see again. Because I’m also in one of those subscription things where you actually don’t have to be member of a specific gym anymore, you can just go anywhere they partner with. And I think lots of people are doing that nowadays, plus with all the layoffs people move around even more than they used to.
I think phones do play a role but they aren’t the only thing that keeps us from socializing.