I recognise the last two posts. I think part of it is who we are and part, at least in my case, is childhood experience.
I recently read an article by someone, who like me, had a peripatetic childhood, because her father was in the forces. She had only been to 4 schools, I went to 9, but she commented that you become very good at socialising with anyone and everybody but do not build close relationships because you know your time there is limited and that you will then move elsewhere in the country or overseas.
In my case this was exacerbated by having, until I was 13, a medical problem that left me vulnerable to bullying. I can talk to anyone, anywhere but find it difficult to step over the threshold from acquaintance to friendship. I do have friends but they are all 'love at first sight' friendships, we met and bonded, and as they are all very outgoing I was stepped over to, rather than making the move myself.
Having said that, even with people with large social circles, the number of close friends they have is probably fewer than they would admit. I have a long standing hobby and I have belonged to the local group associated with it for over 30 years. I am very friendly with a good 10 or more members, we meet regularly, have a regular café where we have lunch before meetings and email each other regularly about group activities, but I do not socialise with any of them on a one-to-one basis - and I do not think other members do either, but if someone went missing or we didn't hear from them for a while, we would all soon be emailing or phoning to find out what had happened to them.