Ooh, good question. I'm mostly not a partier, though the reason why has varied.
For most of my 20s and into my 30s, it was because I was both working a job that required being there early (school library assistant) and because the times I was out doing stuff were mostly (religious witchcraft) rituals, classes, and events and I didn't have much spare time left over. So the more 'party' stuff was limited to things like the occasional festival, SF convention, or special event and even then, I ran out of ability to people before midnight most of the time.
These days, health stuff means I limit how many things I do away from home in a given week besides work and the gym, so I mostly pick seeing people I know and like either individually or maybe in a small group, though there are definitely exceptions. I'm a lot more willing to risk 'maybe this social thing won't be a great choice for my time' online where the physical costs and limitations aren't such a big deal.
Re: Socializing when you're not a partier
For most of my 20s and into my 30s, it was because I was both working a job that required being there early (school library assistant) and because the times I was out doing stuff were mostly (religious witchcraft) rituals, classes, and events and I didn't have much spare time left over. So the more 'party' stuff was limited to things like the occasional festival, SF convention, or special event and even then, I ran out of ability to people before midnight most of the time.
These days, health stuff means I limit how many things I do away from home in a given week besides work and the gym, so I mostly pick seeing people I know and like either individually or maybe in a small group, though there are definitely exceptions. I'm a lot more willing to risk 'maybe this social thing won't be a great choice for my time' online where the physical costs and limitations aren't such a big deal.