What economies of scale are even possible here? California in particular is relatively isolated from the rest of the country because the thing directly to the east of it is a major desert, followed by a sparsely populated mountain range and then a very large amount of farmland.

The nearest city to California with more than a million people is Phoenix, AZ which is "only" a couple hours from the California border. The next nearest is San Antonio, TX. The distance between Phoenix and San Antonio is about a thousand miles. Neither of those cities themselves have a functional mass transit system for anyone to use even if you put a rail stop there.

California itself constitutes more than half the population of the entire western US, which is otherwise enormous with a very low population density. It doesn't make sense to put high speed rail anywhere in the western US outside of California because there aren't enough people there to use it.