The law doesn't mandate building apartments either, so if the same people who build/buy houses as rental properties are unwilling/unable to invest in apartments (because building a 400-person complex is a lot more expensive than a house), then rental property supply can only be constrained by this law or concentrated into the hands of management companies (who have a vested interest in constraining supply)

Furthermore, I'm not completely convinced that restricting rental property ownership to property management companies who can afford to build apartment complexes is inherently more equitable than letting a middle class person with good credit buy an extra house and rent it out. Where I'm from originally (Alberta), it was common for electricians/plumbers/tradesmen to buy an extra property with a combination of credit and wages and maintain it. It's a relatively simple and time-tested path for moving from working/lower middle class to being upper middle class or even wealthy.