"Someone presently in the valley, who is worth attracting, generally has no reason to leave" - I get your point but many people once they have families feel pressure to get out of SV probably due to long hours, cost, etc. Not everyone leaves but I've seen many good engineers leave to the more affordable tech hubs (Austin, Portland, Seattle, etc.) I have to admit that once I started visiting these places I can now understand why many people move.
why would you move your family to an area with less opportunity for them? Is their partner going to be able to find a "propper" job will their kids get a good education and have a wide choice of "propper" carrears (or will they join the army as thats the best choice on offer)
Its probably worth saying that there is a big difference between moving from SV/SF to Seattle, Portland, or Austin, and moving to a small town of 1000 people in the middle of nowhere.
Opportunity for career growth isn't the only factor in where a family moves - there are a number of other concerns like safety, values, closeness to family, and fresh air and water.
