It isn't always. But making just about any decent mass housing, especially affordable, relies entirely on state and federal grants and tax incentives to defray the costs and make up for the reduced revenue. Basically developers will want to see a certain amount of money more or less guaranteed before they're on board with something - and typically they can't do any differently even if they wanted to, since despite sustained low interest rates and massive quantitative easing, banks are still really tightfisted about loaning money and large construction projects are very, very expensive.

All that to say, you can sometimes find good, new affordable housing, but it requires very particular incentivised sites (historic, brownfield, et al) as well as direct government support for affordable housing itself. Also you have to avoid any public backlash. So it's possible and it happens, but it requires someone to take on a lot of work and be pretty creative to not necessarily even make as much money as they would with a regular apartment shitbrick.