Vacancy rates are very low. There just aren't that many unoccupied homes. Even these companies buying property are largely renting them out. They aren't sitting empty.

The ability of these companies to make money is premised on a low supply of housing in high-demand locations. If housing supply increases, both market rents and housing values should fall or at least track below inflation, leading to these companies putting their homes on the market.

There aren't enough houses. Boston has been adding high-paying jobs like crazy, and those people need to live somewhere.