According to Reason, regulations and bureaucrats:
In an alternate reality, the city government wouldn't be calling shots on what is legal and illegal in terms of apartments, rents, sub-contracting, you name it. That would fall on the shoulders of owners and renters rather than what is at best a capricious set of rules enforced by bureaucrats whose actions are subject to wide variation.
According to Lisa Sturtevant of the National Housing Conference, regulatory approvals (zoning, inspections, and more) add up to $50,000 to the cost of new single-family dwellings in urban areas. That's a lot of scratch that ends limiting housing supply and squeezing residents in all sorts of ways.