I think some people are just the center of gravity, and that particular friend circle revolves around them. Before COVID, we had a friend group that would hang out fairly regularly. Once I left (for a job, not fleeing the city), none of them hung out without me. Everyone was friendly with each other, but everyone also had their own lives going on with their own friends and other circles. While I was the glue for that circle, it wasn't like everyone just stayed at home having pity parties when I wasn't around.
My anecdote might have limited relevance here, but I think it's something worth considering.
Do any of those friends organise anything for any of their other friend groups?
I get the feeling that some people organise, while most people don't. I haven't seen the situation where a person organises stuff for one group, but not for another group. It always tends to be the same people doing the organising for all their friends. At least that's what I've observed, I'd welcome any other observations.
edit: poor choice of words
"Most people have less than the average number of friends", or The Friendship Paradox: