It boils down to a limited supply of permitted lots. It takes a long time in most counties in California to get through all of the approval processes, impact studies, regulations etc. A lot of the same people who want affordable housing regulate for ‘smart’ growth which is a code word for low or no growth. And by affordable they don’t mean cheaper…
Developers make higher margins here on the land that gets built in order to compensate for all the projects that never go anywhere. Texas is a pro growth state where you can get land permitted to build on in a few months, something that can take years and year here and sometimes it never happens.