Every year, the School of Computer Science publishes a survey of where undergrads go after college: https://www.cmu.edu/career/salaries-and-destinations/2015-su... [pdf]
About half of the 2015 graduates go to employment in the West/CA.
I'll add that I am not even a graduate in CS (I majored in Business), yet I know a lot of non-CS majors my year who have also moved out here.
California is where the prestigious jobs are. And that's what matters at the end. Is Pittsburgh capable of supporting a Silicon Valley? Not as long as Pittsburgh's weather remains the way it is, anyways.