Why? Because apparently they need some more incentive to keep units occupied. Also, even though a property might be vacant, there’s still imputed rental income there. Its owner is just receiving it in the form of enjoying the unit for himself instead of receiving an actual rent check from a tenant. That imputed rent ought to be taxed like any other income.
I get what you’re saying. I agree that there’s no new tax policy that will fix the landlord problem on its own. Ideally, it would have to be combined with a massive build out of publicly owned housing and supporting infrastructure, especially transit.