Silicon Valley has the lowest latency in searches, both for job seekers and for employers. If you're looking for someone with deep experience in 3 disjointed areas of technology, you can find that person in SV. You can't easily find that person anywhere else. You have to train someone into the role, or convince the one matching person in the world to move.

For extremely specialized queries, SV still has a continuous labor market while the rest of the world is discrete. If you're doing the kind of get-big-or-die red-ocean gambit that VCs like to fund, that latency risk is undesirable.

Other than that, I think SV is overrated. There are probably 20 places in the country (many cheaper and with fewer douches) where it's not hard to find generally smart, technical people in large numbers.

Anyway, I can't imagine anyone under 45 willingly living in "Silicon Valley" unless that includes San Francisco. The reason for moving out to the suburbs is to get a cheaper place and more land, but the Valley doesn't have that going for it because even suburban houses with disastrous commutes are expensive.