I've observed children who have had tremendous close friends in childhood but were unable to recreate that in adulthood. Sometimes it's easier to make friends when you're 5.
Adults actually have to work at making new friends and few get any tips on how to go about doing so. I honestly didn't really get any until my late 30s or 40s - and that was mostly because I moved to Norway and for some folks, loneliness and lacking connections is a real issue.
Children have school. School gives you a shared experience to talk about and time to talk to others, both through actual coursework and play. Children are handed the tools to possibly make friends and they aren't even old enough to have decades of baggage and anxiety yet.
As an adult, you have to create those conditions. For many, work serves this role. Hobbies and regular activities (bowling, for example) help. Depending on the person, it can be online (Met my spouse this way - a silly online game back in the later text-based, formulaic MMORPG era). And you are a lot busier as an adult with more responsibilities filling your time. Of course it is harder as an adult.
I got a lot of flak for going to a high school play in my late twenties. I had played D&D with the kid and his mom every week for years. He was great to hang out with when he was 14.
The whole country is engrossed in a decade+ long "pedo panic" to the point where you can't support a friend's kid by attending a school play, take them for ice cream, or (sometimes) even take your own child to the park without getting the side-eye from nosey nobodies.
Kids in that age range are uncomplicated. The only thing they really desire is that you play with them. They just don’t consider anything beyond that.
But it seems hard for many adults to play with children, so it becomes this anomalous thing, even though I’m fairly certain it’s just something we’ve convinced ourselves adults “don’t do”.
