Cue the standard responses about lack of development, rampant NIMBYism, Prop 13 and rent control. There's really nothing to add at this point. All of this is also why I really prefer living in NYC, which actually has the infrastructure for this sort of thing (meaning if you're a lower to middle income earner in the tri-state area you're not totally hosed like you are in the Bay Area).
Higher up I posted a list of actions we can take today to help make housing more affordable in the Bay Area.