What happens when the homeowners can’t find any buyers? Or the apartment owners can’t find new renters?
A very large proportion of that sort of property in SF (and elsewhere, Vancouver tends that way too), is a pure real estate investment, as a hedge, or in some capital flight scheme. There are many condo buildings that are mostly empty, with foreign owners (mainly Chinese) of most of the units. Many of those people dont want to rent their units as that means they are that much less liquid.