It's not only in silicon valley. My first company (in Switzerland) was like that. It was a start-up, mostly with people in their 20s fresh out of uni, no family and often expats/not from the area. Basically, people without their own life there, so of course colleagues become friends, and people recreate the university lifestyle they've just left, except this time you're getting paid. I think the appeal is clear if you're in that situation.