>To be fair, developers sell units at what the market will bear, rather than on a "cost-plus" basis. Raising developer costs doesn't impact the final sale price.
Raising developer costs kill off unprofitable developments until the marginal unit created has a cost equal to what the market will bear. It depends on the relative elasticity of supply and demand - we'll hit an equilibrium of fewer units at a higher price, with both less developer profits and less consumer surplus. Whether the loss is mostly borne by purchasers or sellers depends on details of the market.
>but it's properly zoned land, not developers that is in short supply.
Developers in the Bay Area spend lots of money to figure out whether or not land is properly zoned, due to the community review system and other things that'd be fixed with a by-right zoning policy. Things aren't built to the maximum of the zones - they're built to what's affordable by developers attempting to get permission to build.
>given the scarcity of land it doesn't make sense to build affordable housing unless forced
It doesn't matter whether housing is affordable at market rates because we build things that are affordable, or through building so much luxury houses that there's so little demand for existing buildings that they now become affordable.
If the lot next door added a three-bedroom luxury apartment that cost less than your studio, you'd move out of your current place and into that in a heartbeat. Then your studio now goes on the market and gets whatever it can - likely, a much more affordable price.
Adding dense housing makes housing more affordable. It's only obvious if it's cheap and dense - if it's expensive, the affordable units are those that the new residents move out of.