How did you come to this conclusion? If someone is renting it means they they can't pay for mortgage. Otherwise they would've done so. He said, that he needed to make a 20% payment to even get the mortgage. Idk how much money that was for him, but where I live that would be around 130k$. Clearly not everyone has that kind of cash.
And what's your solution? Disallow renting properties for which mortgage wasn't posted in full?
Let's say the mortgage is payed. Then the rent pays whatever the landlord decides to do with that money. Like literally any other transaction.