Because NIMBYism, height limits, and single family zoning over most of the landmass still exist, and have much stronger effects.

They are in the present universe, where we don't have effective rent control.

And effective rent would require controlling evictions and controlling rents on vacant units (infringing further on sacred property rights, certainly).

Now, the most effective policy would involved rent control but with exemptions for new very high density units, incentivizing increases in the total housing stock for an urban area.

But we're here with weak rent control and tacit collusion of homeowners and builders, who talk about their property rights when convenient and use the state to keep their monopoly when convenient.