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.
Another term is a vacant homes tax, something Vancouver and Toronto in Canada are using.
These sorts of narrow, “feel-good” taxes are the wrong way to go. People find loopholes to avoid paying them.
Georgist land value tax (LVT) is straightforward and cannot be avoided. It incentivizes owned land to be utilized, otherwise it becomes a huge liability. It does not disincentivize improvements (building stuff) because taxes are tied to underlying land values, not improved property value.