It is a matter of supply and demand, and while demand in SF is high, it's nothing like say NYC or LA where there are millions of people looking for housing. SF is a medium-sized city as far as cities go. It's a growing city, unlike many, but in the last ten years it grew slower than Boston, DC, Portland, Seattle, San Diego, and Miami. It's not like developers can't build housing quickly enough to keep up with all the demand. It's that the city won't let them.