We're not based in SF yet. The reasoning for going is unromantic: that's where our buyers, our future hires, and the people who've already done what we're trying to do all happen to be.
There are good reasons to build a company anywhere. There are very few reasons to build it far from your customers.
Inspiration is a real input too. You absorb the seriousness of the people around you. In SF, the bar is set by people aiming higher than you, by default. That's worth a plane ticket.