Case in point: Los Angeles, which, on the basis of median income, has the least affordable housing of any major city in the US, also has a significant number of vacant apartment listings.

The rental rates for these (mostly new) apartments are so high that they don't address the shortage of housing in the low to mid income range. And the owners (corporate, mostly) prefer to keep the units empty instead of lowering the prices.

So, in a sense, Los Angeles has both too few and too many housing units.