Because in my experience, where I live, once people past 30 have at least one reliable friend in their life the can rely on, they don't open themselves to making new ones, so they invest their time and energy elsewhere.
One person was really blunt with me: "you're a pretty cool guy, but I don't have time/space in my life for new people". Other people are less blunt but the same principle applies.
Obviously I can't say for sure, but to me "you're a pretty cool guy, but I don't have time/space in my life for new people" sounds much more like a polite excuse to avoid saying "you're not somebody I want to be friends with" than an accurate statement about the reason they can't be friends with you despite honestly feeling you and they are compatible friends.
edit: But maybe that's my British instincts wrongly diagnosing a statement that I have no real context to judge better.
Quite possibly not. If you have job, family, friends you hang out with and one hobby, then there is often zero time for new friendship. Cause that would require additional time.
Maybe, maybe not. Someone I know who seemingly leads a normal social life turns down all my invitations to hang out with groups because he "doesn't need to meet new people." And he's not the only person I've known who has that attitude.
Sure, that's why I said that I can't know for sure - because it's a sentence that can be used truthfully or it can be used as a polite lie.
I just feel that if it were possible to track every time that sentence is said it would probably be significantly more often used as a polite excuse rather than as a truthful statement. (And even then, that's my gut feeling / assumption, I may even be wrong about which use is more common).
One person was really blunt with me: "you're a pretty cool guy, but I don't have time/space in my life for new people". Other people are less blunt but the same principle applies.