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.
It does exist, but it’s not obvious because it’s income in the form of the rent you don’t have to pay because you are both the owner and possessor of the house. It’s kind of like how, in the USA at least, if I get free tuition because a parent or spouse works at the university I attend, I will owe income tax on the tuition I’m not paying. Or, if I have some debt that gets cancelled somehow, I will owe income tax on that amount.