I've always assumed it was the lack of BIG things happening that limits the depth of friendships. With my childhood friends, we went through so much growing up together (family deaths, relationships, breakups, make ups, divorces, death of classmate, arguments, school, sitting around together for hours just hanging out, etc.)

Those sorts of events really deepen a friendship. I don't experience anything like that with my adult "friends". That limits how friendly we can realistically be. Sure, I've made friends as an adult, but they're just people I know and enjoy seeing once in a while.

Maybe having really deep relationships as a child makes it that much harder to make friends as an adult as they suffer by comparison.