The majority of people who vote in elections and show up to city council meetings want housing to be an investment that grows in value by 10% annually. Building more houses is a direct threat to that investment. Guess which group wins.
(Caveat: California's state legislature is slowly clamping down. Regions which aren't submitting realistic plans to meet their projected housing needs are getting their zoning privileges taken away, which will help. However, this is like trying to empty a swimming pool with a teaspoon when it comes to the supply and demand mismatch that exists today.)