This is largely due to politics. If you were a speculator considering building a rental apartment building, you face:
1) Rent control. (Those cities and counties that don't have it are all considering it.)
2) Requirements that you allocate a percentage of your units as limited to "low income housing", with those units becoming permanently constrained
3) permit costs, neighborhood opposition, and other city-specific barriers
4) high material prices and construction costs (e.g. earthquake and flood codes)
5) high labor costs
This is largely due to politics. If you were a speculator considering building a rental apartment building, you face:
1) Rent control. (Those cities and counties that don't have it are all considering it.)
2) Requirements that you allocate a percentage of your units as limited to "low income housing", with those units becoming permanently constrained
3) permit costs, neighborhood opposition, and other city-specific barriers
4) high material prices and construction costs (e.g. earthquake and flood codes)
5) high labor costs