And I believe the census goes by what the property owner claims. “My 120-unit condo building with nobody living in it? Yeah, that’s not housing, Ms. census taker.” “Ok.”
And even that doesn’t include industrial, commercial, and office properties that should be converted to housing in response to fair market demand, artificially withheld to make them more liquid as stores of value. The town is full of these and few people notice because they’re not housing and they’re not for rent.
As for commercial space that's not housing, that's up to the city government to change, and the city government and resients who show up to planning meetings oppose that sort of rezoning. This being a conscious action of owners of industrial and commercial property. Are commercial sites more valuable than residential right now in SF? What sort of use brings these owners more gain than renting our properties that they are legally allowed to rent?
Remember the Ghost Ship fire? The proprietors were charged with involuntary manslaughter for using a warehouse as housing:
https://en.m.wikipedia.org/wiki/2016_Oakland_warehouse_fire
I guess I'm just very confused about what you consider to be very obvious. I can't see evidence of people hoarding buildings that could be used for housing. I do see SF residents preventing more buildings being used for housing, and preventing the building of more housing on infill sites.
SF's population has increased 10% since 2010, and I don't know how many people have been priced out, but it's probably at least on that order of magnitude. Held off existing properties don't make much of a dent into that delta, and it's unclear how to lower them even more if they're already so low. It's also unclear why it shouldn't be allowed to be brought up.