They could probably start by putting all the units they’re holding off the market back on the market, but that wouldn't let them keep rents artificially high.
And if they did that then they couldn’t exaggerate the value of the buildings, since the theoretical earnings is based on the average rent multiplied by the total number of units.
And if the value of the buildings dropped because the average rent fell, then they would suddenly be over leveraged on all the loans they’ve taken out with the buildings as collateral, and they couldn’t take out new loans to pay the interest on the old loans.