There’s a great discussion on Quora on, Why is there such a stunningly short supply of designers in Silicon Valley right now?
All the responses are insightful, including Mike Davidson’s:

This is not a supply problem. It’s a demand problem. There are more talented designers in the world right now than there have ever been; more than enough to supply all of the startup ideas that are actually good ideas.
As well as Willie Lim’s:
Designers are not only short in supply because of demand. Designers who can cope with the quick, ever-evolving demands of the technological landscape are rare to begin with.
The technical skills required just to be considered for a visual design position are high in today’s world and the positions available in the Valley aren’t always compelling to the sophisticated designers they’re looking for.
The elusive “rockstar” designer you’re looking for isn’t interested in playing on piece of wood with a few wires stretched across it. Unless of course you’re Jack White and then you start your own band.