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.
In the case if vacant rentals it’s actually kind of easy since the landlord will have advertised the rent amount. And therefore, there’s a lot of data out there as to how much could be charged for so many square feet and bedrooms, probably more than what is available to estimate property values.