NIMBYism alone also isn't the problem per say, it's zoning plus NIMBYism that makes things hard. A vocal minority can effectively block zoning changes, which prevents developers from building high density housing. This is exacerbated by CA's so-so to terrible public transit. High density housing is far more annoying for whoever lives nearby when everyone living there has a car or two and the builder didn't also build a parking structure for those cars.

This is mostly a problem for coastal California too because land is limited and expensive. In inland California a developer can just buy and build someplace else.