To reduce transmission losses, among other reasons.

> California has vast desert areas to tile with solar panels at less expense.

Even if it would be less build/maintenance expense per unit of delivered energy, a lot of California's vast deserts are either federally protected, and so not available even if it was smart to use them otherwise or already being irrigated and used for agriculture. You can't really cover a farm with solar panels without killing it's function like you can a house.