They didn’t know that it would probably become an airbnb, but the likelihood of such a place becoming one or being bought by somebody else wanting to charge higher rents isn’t exactly low either.

The whole “market rates” thing is used by corporate landlords to increase prices, and controlling available properties - including by leaving vacancies - is one way that do that. By the same token, charging below “market rates” could also help of enough did it, especially if the places are decent and money re-invested in proper upkeep. Many/most though are not even investing in proper maintenance/repairs while charging over an above the cost of entire mortgages, which IMO is just greedy bullshit