I think some people do this. However, I’m 30, live with my long term partner, and have a bigger friend group than I’ve ever had before, with weekly events. My partner isn’t a stand-in for socialization.
I’ll fully admit I have some advantages because I have no kids, and a job that pays decently and isn’t too demanding. I’ve met people through:
dating apps. This is how I met my partner and also a very good platonic friend
activist/interest groups. Got involved with a local urbanism group, now I know many of the people there
house parties. I got lucky here, I met someone that throws monthly house parties, went to those regularly, and made some very good friends that way
reconnecting with childhood friends. Again, lucky, but a few of my HS and college friends live in the same city as me and we reconnected and hang out.
The one bit of concrete wisdom I think I have here is that if you go to the same social place regularly you’ll see the same people and if you put yourself out there you’ll get to know some of those people. Activist groups or meetup groups are great because you probably already have some things in common.