If property taxes were evaluated the same way for the landlords as for everyone else, that wouldn't be feasible would it? And home maintenance and insurance for unoccupied homes is typically very high. There is also the risk of squatting or the cost of people to prevent that.

Has anyone done some math on the housing density that is most economical? My guess would be that the height limit would be close to five or six stories before the engineering to go higher was excessive, but that is just a wild guess. Courtyards down to ground level so people can have an external view seem wasteful from a heating and cooling perspective, most people don't want to live with no windows.