A non-trivial amount of "property" is speculation at best and concrete money laundering at worst, especially in the largest cities

It's not supposed to saleable or rentable. But it does take up land that could be used for real housing. And it also drives up the perceived value - and price - of both rentals and outright buys.

Rent control may or may not be a solution. But many of these schemes are run through shady shell companies, and aggressive enforcement of financial reporting and taxation would go some way to fixing the problem.