I think more than anything it's the amount children share in common. Adults are lame. The world is big. There's a lot to do and a lot of time to do it in. If you're a kid and you meet another kid odds are you live in the same town or at least region, go through the same school system, are roughly socioeconomically similar, and (thus) have a lot of things you can do together.
At work I really don't know much about my co-workers. I would go grab a beer and watch the game with someone if I knew they were interested, and from there we could hang out more -- but I don't. We spend all day near each other, at the EOD I just want to go home.
So my theory is that as adults most of us have fulltime jobs where we're always near people we don't spend much time with, and have no time to enter social circles afterwards.
$0.02