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.
Why not simply take the property from the “landowners” and possibly even jail them for being a leech on society? If you own property that you don’t live in, you are taxed at 100% of the property value if it’s not occupied. If you declare bankruptcy you immediately forfeit the property. Easy.