When I was in the Valley a few years ago, lots of people were quite convinced that top talent had already been completely chained to Google/Facebook/Twitter anyway, and there was no chance of poaching it away. If you follow that line of reasoning, there is no point investing in recruitment, at least locally.
I also wonder though if the real estate hyperinflation of the valley is a factor. It negatively impacts the ability to attract out of town talent when a "starter home" is one million dollars and a decent apartment is over $3000/month. It also impacts retention. I meet a lot of folks from the Valley and SF who loved it but "left cause they wanted to start a family" or "left cause they got tired of spending >50% of their income on real estate." You have either pay exorbitantly high salaries or expand elsewhere.
Manhattan is similar to SV, but the thing is this: it is possible in New York to find reasonable (for the local market) real estate within a reasonable commute from work in one of the boroughs. It won't be the trendiest, but it's going to be okay.
In SV you are talking about $800k or $2500+ a month for places that in other parts of the country would be called a "crack den." Either that, or you are going to be both far away and in an utterly dull, depressing suburb.
SV is America's only six-figure ghetto. There is nowhere else where so much buys so little. Driving around on my most recent visit I was shocked by the squalor of these six-figure engineers and millionaires (relative to what you'd get for that price in sane markets).
I'm not a local so I can't say for sure, but I'd be strongly tempted to blame the anti-development political mentality. With growing demand and no supply, this is gonna happen.