CA's large GDP is the result of the diversification of its industries. CA has the largest agricultural industry in the U.S., the largest manufacturing industry (yes, people still make things in the U.S.), the largest entertainment industry, and the largest tourism industry.

And that doesn't even take into account the purely local-market GDP in the Bay Area, LA/OC metro area, or San Diego, each of which boasts a local GDP larger than the total GDPs of most of the Midwestern states, or, for that matter, industries where CA has a large market but isn't a national leader, such as finance, biotech, or resource extraction.