“land banking is economically unproductive, and there isn't much dispute about that”

I think you are confusing current utilization with economic productivity. There is an opportunity cost in hoding the land but land can also have economic value in the option. It’s the owner choice if he values its use even if it’s waiting for the option to come through and values it more than whatever alternative a self professed smart politician or OP’s abstract “society” might propose.

Whats’s next? Taxing you for your unused bedrooms, unused cars, unused clothes in your closet, unused nose rings, etc etc?

Just because some policymaker can identify a more productive use for someone’s property does not mean the policymaker knows the owner’s opportunity cost or future options or even risk better than the land owner.

An even better argument might be why do we need to tax at all? It’s not that governments are the pinnacle of capital allocation efficiency. After some useful level, governments are basically corruption machines ESPECIALLY IN CALIFORNIA. Just look at their homeless and illegals industrial complex.