I dont doubt there is a point where buying and holding allows for such low supply that you can run up the prices (see the global diamond market). But, i think you have to own what amounts to a monopoly-share of the market for that to happen. So its gotta be something else.
In RE, its the land that appreciates, not the house. And renting the house out guarantees revenue, without affecting the value of the land. In 10 years the house itself will be outdated and need a remodel whether or not it has been lived in. And renting the house out takes the house off the market so supply scarcity is the same as leaving it empty.
I guess it might be that institutional investors bought houses with cash, and then counted on being able to finance remodels/renovations... but are not able to make it pencil with the current rates (or their umbrella companys current balance sheet). So they are caught with this deadstock inventory having not planned for a Plan B, so they are hoping that any month now they can secure loans to fund the remodel/flip. And when you are living month-to-month on maybe remodeling or not you cant really rent it out.