As crazy as it is to hire in San Francisco, hiring in the Valley is several times harder. Being in downtown SF, I really don't have to compete with Facebook and Google for engineers, especially anyone coming from the North or East Bay.

Add in a huge amount of local design talent, a city that people outside of California want to live in and would be willing to move to and engineers that are generally happier/more well-rounded.

I honestly can't come up with a good reason to be down in the Valley.