What makes you think that people aren’t moving? Population isn’t decreasing if that’s what you mean, but the Bay Area is a revolving door. I own a condo in San Francisco, and I’ve had five different tenants in eight years.
Of course plenty more from later years offset that.
My (bay area) college class was half from CA, half from the northwest. About 50% of those from the northwest moved 'home' after school. Is that a revolving door? Did they get sick of the area? Or did they never intend to stay?