> why wouldn't established companies like Stripe just ... off to another city entirely?
Because the employees they want to hire live and work in Silicon Valley. There's a lot of advantages to the area: Good weather, good cultural actives, good jobs...
Keep in mind that companies loose people when they move within the same geographic area. (For example, someone commuting over the Golden Gate bridge might decide to leave because now the commute is much longer.)
* By the time I screened with them, I had a job offer in hand that I really wanted. (This was long before Stripe became successful.) I lived a few blocks away, so I asked to screen in person, and then decided to take the job with the other company. I don't even know if they would have hired me!