In my experience, the vacant housing is not built without demand, it's that the demand vanishes.

There were two trailers where they would have been scrapped, but some relatives took then over and kind of refurbished them, and one of those is now home to another relative that would have been homeless otherwise, and the other is a "hobby" trailer until someone else needs it.

Another is a house where the man died and the wife moved to a small apartment because she felt like she needed to be in the city near a hospital, but no one wants the house because the area is the middle of nowhere.

Rural areas tend to have a fair amount of "nobody wants them anymore" housing laying vacant, but they all, at one point, were being used as housing.