[ausbah]

landlords rarely build housing, they usually buy up existing housing and rent it out

and the don't have to do anything illegal to restrict the housing supply - they can lobby, regulate, and vote

[chmod600]

I don't know what you mean "rarely", because clearly a lot of housing is built and intended as rental property.

Regardless, the small landlord who buys a single family house and rents it out still puts a new house into the rental market, making it more affordable than it previously was on the purchase market. Along the way a landlord typically makes improvements and pays for maintenance, which improves the supply.

If you don't like laws then complain about the laws. The act of renting a house to someone else is not the problem.