Are you implying that landlords are naturally incentivized to build homes? Because in most circumstances, the exact opposite is true. In the U.S., the government has a number of programs that offer landlords vouchers in order to encourage them to build out more homes.

Landlords are different things from developers.

Sometimes (more rarely than people think) a single entity plays both roles, but it's impossible to reason about this space if you conflate the two

Yes, developers build homes to make money. This is how approximately 100% of the housing supply in the US was created.