Which is even less explicable to me. I get along with my coworkers, but I don't hang out with them after work because I have my own life and work is just the thing I have to do to support living it. Why in the hell would you want everyone you work with to be your drinking buddy? And what sense does it make for a company to give up access to all that experience for that?
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.
SV business culture is truly insane.