Other people, you become friends with due to some shared situation. School, work, place you live, the places you go/frequent, etc. Once that changes (you graduate, switch jobs, etc.), your friendship can change.
And, of course, your other responsibilities will influence how much time you can use on people. Especially children can have a huge impact on that - kids just take up so much time, and combined with work and other things, it is really difficult to prioritize other things. It is not at all uncommon that once people get kids, they disappear for a solid 5-10 years, and will want to catch up again when things calm down.
As young adults, most people have few responsibilities, and impulsivity is high. What I miss about being a young 20-something was how easy and willing everyone were to do stuff. Go on a hike, go watch a movie, go on a pub crawl? Sure, just give me 15 minutes. Book a trip to some other country? Could do that just a couple of weeks ahead.
These days you'll have to check your calendar 3 months in advance to just shoot the shit.
I think the change started when me and my friends started nearing 30 / late 20s. That's when people were really bogged down with work, met their future spouse / partner, and started focusing on self-realization (working out, hobbies, side hustles, whatever), and of course - kids.
Now that most of us are in our late 30s, things are a bit easier. Those that got kids have more spare time, as the kids have grown older. Seniority at work means they aren't giving it all for the sake of promotions. More financial freedom. Things more stable, and people can catch up again.
With that said, some days I really do miss the days of youth.