You have to work at it. In the last month, I've met with school friends, ex-colleagues, former housemate, family. Have also been in touch with uni friends and overseas friends - coming up to Christmas helps, to be fair.
But I have a personal rule that I don't turn down invitations unless I'm already booked or it's something I really won't do (and currently can't afford, as I've no income right now) - no point taking me karting or to a fish tasting menu, for example.
It means I I'm travelling somewhere, I'll usually ping someone who lives nearby or en route to see if we can manage a meet up - especially as i luve in a town people don't tend to visit for fun. It also meant I accepted as a childless person, I was probably going to have to do more travelling while friends had young children (things are improving now we're of an age where most of those kids are teens or older.)
I also have various local things I'm involved with - regilar sport, language lessons and so on. Obviously parents will have less free time, but it opens other opportunities- at my mother's funeral, there were people from the baby sitting circle they'd had about 35 years earlier, other school and swimming club parents and so on.
I sometimes wonder who would make the effort if I didn't, but it's not an experiment I want to undertake, as I'd probably be the one ending up all alone.