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.
If you own 100buildings, the proper tax burden is that you shouldn’t be able to own 100 no matter what the rent would be. The tax burden should be so high that you are losing money for each building regardless of rented status, until you get down to a reasonable number of homes of say 2. And you’ll pay a decent amount for that 2nd one if it’s in an urban area.