How much do you think moving to the valley improves the odds of a startups success as opposed to another city, say Atlanta or Raleigh on the east coast?
SF talent is on the same level of experience and variability, though if anything it's easier for people to job hop.
But what it really does is fail to capitilize on the assets of an entire nation (or nations, in the case where people from other countries think they also need to move).