Because anyone who's been living in the same place for a while is paying well below-market (by design, otherwise there'd be no point in the rent control). Actually it's worse than that: the market rate rises much faster because landlords have less upside so there's little reason to build, and existing buildings can make more money as offices rather than residential. And when there's a huge gap between what a new tenant would have to pay and what the current tenant pays, that creates a huge incentive to do illegal sublets, and if your sublet is already illegal then why bother spending money complying with fire regulations or any other safety regulations?