And rent control exacerbates it, as you have literally over a million people that are a "locked into" their below-market rate units and can never leave because they'll never find anything that cheap elsewhere, regardless of whether they still need that much space. My great aunt for example is 101 years old and lives in a 3 bedroom apartment in Queens. 50 years ago there were 4 people living in that apartment (including her husband and her kids, all now dead).

Plus there's the issue of renovations/demolitions that remove units and replace them with a smaller number of more expensive units.