The data suggests that many of the most successful companies are there, but certainly not all of them (and the ones outside are probably a lot quieter or covered less in the news). Turns out there is a lot higher concentration of people trying to build startups there than the average city, so as a consequence of that more are there.

Having built my own, raised money, and gained traction, all outside of the valley (though I do spend a fair amount of time there), I don't see myself rushing to do my next one in SV.