Reasons I've heard: - Cost of living relative to expected salary is too low. - Can work for big public companies that pay well in lower-cost areas. - Weather

Reasons I've heard: - Cost of living relative to expected salary is too low. - Can work for big public companies that pay well in lower-cost areas. - Weather preferences. - Family lives thousands of miles away from SF. - Over 30, still interested in doing technical work. - Want to own a home, not a millionaire. - Interested in starting a business, low-cost matters if not going for VC. - Found interesting & challenging technical work elsewhere. Etc. This is kind of like asking why people didn't all move to NYC in the 2000s, or Texas during the oil boom, etc.