Earlier quoted context omitted.
It could be as simple as Bay Area hackers being accustomed to higher salaries and negotiating higher (or not accepting lower offers) even in places with cheaper living expenses.
There is that, and the fact that in the eyes of prospective employers Bay Area experience is highly valued. Honestly I don't think experience in Bay Area is necessarily better, there are great teams in Europe/rest of US and shitty teams in SV, but for some reason there is this Bay Area "halo". Source: I've relocated to Europe after 3 years in Silicon Valley.
My company has software developers in Los Angeles, Boston, Dallas, and Sunnyvale. From what I've seen it's a lot easier to bullshit your way into a senior software role in the Bay Area than it is elsewhere, so the rest of us tend to be a bit wary of the ones we don't yet know well.