Because many of the workers who are "good" come to the Bay Area for the money, the lifestyle, the location, and the job selection. If you could, why would you not want to live in an excellent climate by many of the US tech companies and make 3-8+x what you'd get elsewhere? You also typically get to work on "cooler" stuff since the companies out here can afford to make bigger and riskier bets. There aren't too many AI/ML positions around the rest of the country, but it's hot right now here.