Plenty of companies have 1,000+ developers and zero presence in the valley. It's ridiculously overpriced and only really useful for fundraising. Which is why most successful companies mostly grow outside of the valley.
Yes, there are lots of companies outside the Bay. Yes, lots of them have lots of people.
And yet there's a disproportionate number of companies in the Bay. So perhaps it should be seriously considered that there are rational reasons for that. Ease of access to talent is one I hear a lot from founders and hiring managers.
Have you ever recruited anyone here? The unemployment rate for good engineers is negative here: you're not just competing with their current job, but with the two other startups who are wooing them, and the one they're thinking about starting.
If you're building something that doesn't require elite technical skill (like most startups), locating in silicon valley is a net negative for recruiting. You're going to have a very difficult time convincing someone to leave Google to work on your social secret photo food delivery app for dogs.
I have. I've also recruited elsewhere. The unemployment rate you cite makes engineers much more comfortable with uncertain companies, which is a tremendous stumbling block when trying to hire outside the Valley. It dramatically reduces the risk of joining a startup.