> There's already a cost of keeping them unrented - the loss of rental revenue.
Right. And observing the market shows us that this cost is too low.
Why would I ever sell an empty unit if it appreciates in value faster than it costs me to keep it? Provided that I have no better opportunity to use that capital, which must often be the case.
If you need capital it's another story. But even in case of a single home, the fact that you can sit on it for months instead of lowering the sale price means it's to cheap to keep it if you don't need it.
There's already a cost of keeping them unrented - the loss of rental revenue.
If you've ever tried to sell a house and had it sit vacant for months when trying to find a buyer, you'd find out about that cost.