There is already land tax, but with proposition 13 people lock in low tax rates. So, if you've owned a home for a long time, you are gonna pay a lot lower taxes than someone, who recently bought a condo on land with the same value.
With a land tax two adjacent properties of equal size would be taxed the same, even if one is an eight story apartment building and the other is a parking lot. This strongly incentivizes improving the land to its highest and best use. Under a traditional property tax system erecting that building results in a large increase in property tax (as the value of the structure is now taxed) which dampens the motivation to build. Under California's prop 13, the parking lot probably has a tax valuation from decades ago and thus is taxed even less, further reducing the benefits to redeveloping the property.