2. Because San Francisco is very nice (perhaps) and we have good schools around here. A lot of hires come from UCB, Stanford, UCLA, USC, and other UCs. Those account for several of the top engineering schools in the country/world.
3. Silicon Valley has hit an inflection point where we're now the best place in the world to find TOP talent in tech. It's not a great place for mid and junior level talent, but if you need top talent for an opportunity that is high growth and high per employee leverage, there is no place better in the world.
Ultimately, the main issue is that everything is too nice and worked too well so the cost of living has sky rocketed. And that is driving out everyone that isn't a top tier talent (engineering and opts). It's really sad, to be honest, I love San Francisco and Silicon Valley, and I wish we could find a way to solve the living cost issue and continue to grow as a hub for innovation.