Do you know what would motivate this behavior? Naively, any rent is more than zero rent, so why let a unit sit empty?
There's probably also a fear that if you let a new tenant in for lower rent, this might lower the rent other people expect and the problem starts snowballing.
With commercial units (eg - downtown San Francisco) it is even harder because with remote work, the jobs are never coming back to the city.
There's a substantial cost in having a renter living there, both in dollars and risk. So if the rent isn't enough to cover those, it's cheaper to let it sit unused.
If property appreciation outpaces the gains one could make from renting it out, then it’s better to leave it empty.
It totally makes sense for the rental property owners.