It isn't the valley, it is the gig. I have been at the heart of one startup and part of several others. I grew up as part of a small business. These experiences have led me to the goofy conclusion that the notion of a "job" with regular hours is an artifice. In a startup, you are likely doing something that has never been done before--it is very exciting, compelling, and as PG says, there is just too much to do.

I would say that the valley culture is an effect of hosting many startups, not the cause.