Because housing in this game has long been strangled by the sunken cost fallacy.

They designed a system meant for a relatively niche audience, never having intended for it to be a mainline feature. When it proved exceedingly popular, they scrabbled to keep the same design but adapt it to suit the growing demand. They likely deemed it too expensive or time consuming to start over at some point along the way and thus kept adding onto the resource pit when I new system should have been implemented years ago as it became clear neighborhood housing wouldn't be sustainable.