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.
Holding out for a better deal seems like common behavior for anyone. You describe doing it yourself. However, the consequences of withholding housing are pretty bad. We need policies that convince landlords to cut their losses sooner and either accept lower rents or sell to buyers who will.