The point is that they are plentiful in SF and Silicon Valley, and that reduces risk. When you lose your job in the Bay Area, you walk across the street, resume in hand, and lo and behold there's another tech employer. When you lose your job in, say, rural Florida, you have to move or take up bartending to survive.

I'd love to move back east, and would in a heartbeat, but I'd have to settle for a single digit number of local employers, and that's not a risk I'm willing to take. The fact that houses are $1MM+ here in the Bay Area is the price I pay for a bit of employment security.