For many residents, building more housing and letting more people in would irreparably change the “character” of the neighborhood. This can mean more crime, traffic, and pollution.
Past a certain density automotive transportation becomes unfeasable and people will be forced to rely on public or other alternative forms of transportation like walking or riding a bike.
Reducing housing costs would mean changing the way we build cities,shifting to a European or Asian model. Whereas most Californians are very attached to their way of living.