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.
For as long as I've been alive, one of the lines I've heard is that real estate is always a sound investment. There have also been land taxes for that entire time, most of them being land value taxes. The evidence suggests that the most common form of land value tax, which does not consider how many residential properties an entity owns, is not doing much, if anything, to disincentivize purchasing residential properties as an investment.
A property tax and a land value tax are a bit different: a land value tax taxes the unimproved value of a plot, while a property tax taxes the total value, including the assessed value of the buildings on the land.
One effect of property taxes is that a parking lot downtown pays a fraction of what an apartment building next door pays. With a land value tax, they pay the same, which discourages land speculation by encouraging efficient uses of land.
And we've certainly never gone as far as Georgism, which suggests a land value tax as the main or only source of government funding, set to be around what an unimproved lot on the same location would lease for.