Assuming you're talking about the cities and the coast and not, like, Modesto or Bend or whatever, it's because it's a desirable place to live.

As someone who grew up in the central valley, I don't entirely disagree. I know so many people who are desperate to move to the West coast and are giving up everything they have to get there, and it's funny when they do get there and discover that most of it is dry and hot and poor. The development of high-speed rail and its effect on the housing crisis in the bay area will be interesting to watch, along with climate change dropping its big fat ass right on those snowpacks.