Assuming you’re using “overpopulation” to refer to California’s housing shortage, this is not a “too many people” issue or a “not enough land issue”, but rather a “land isn’t being used efficiently” issue. Efficiently housing and moving large numbers of people is by and large a solved problem, with plenty of examples to learn from (see: any megacity in Asia).

California hasn’t solved the issue because some percentage of residents or other interest groups don’t want to solve the issue and have had the political means to block attempts at resolution.