I'm not sure it's just specialization. Outside of hands-on engineering, there are a lot of roles in product management, developer relations, marketing, finance, etc. that are pretty transferable between companies. There's just a lot of job hopping in SV--especially between often short-lived small companies--that you probably see less of elsewhere. Personally, aside from one company that went pop in the dot-com era--I've never been at a job less than about 10 years on the east coast.

For the Bay Area specifically:

1.) Small startups die often

2.) People vest their RSUs and realize their employer doesn't value them enough for a refresh so they take a giant pay cut hence start the job search