momentum? I have family in tech, and 2 are in San Jose, 1 in New Jersey, and another in Atlanta (a fourth left the industry).
From what it looks like there, they ended up where they are by momentum and job opportunities. Once you have a couple kids and your life is settled, sure moving to another city is in theory ok, but it can be hard if you have a house, schools, etc. It seems moves from one city to another were mainly dominated by loss of job and no options in the local area for that skill set at the time.
I think it's like asking why are investment banks and trading floors located in NYC. It is patently absurd when you could locate in Florida or Texas with much lower taxes, lower costs, etc and almost all the work is done on computer over the internet. But 150 years of momentum (from the days when stock broking was the job and working on an exchange meant something about location) the last 15 haven't seen large scale exodus.