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.
Empty housing is empty land. It just has a house on it. And there are times and places where landowners will spend decades sitting on infill waiting for land values to go up. Additionally, land that could be developed into high density housing but is being held at low density at the behest of the area’s politically connected residents, is kind of like “empty” land. It isn’t a binary.