> 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.