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.
Sounds like a nightmare to oversee and upkeep. Much easier to just make it easier for companies to build more housing so that there’s enough competition to drive prices down