[TomMckenny]

In here are two reasons[1].

Another is this: a building nearly full rented has an easily calculable valuable based on its income. A building with a higher price but not trying to rent can, incredibly, claim to have a higher value. Especially if there are several building nearby claiming the same value.

If there is a debt associated with the building, it is especially in the owner's interest to keeping the supposed value of the building high even if they are loosing revenue at the moment.

If a few organizations own a large number of units they can strongly influence a market both of rental rates and building values by doing this. Undoubtedly to a net profit or they wouldn't be doing it.

[1]https://news.ycombinator.com/item?id=19427243