Earlier quoted context omitted.
>Rent shouldn’t be anywhere near comparable to mortgages. How am I supposed to rent you a house if the rent you pay is less than the mortgage I have on the house? How do I build a pool of money to use for repairing the things that inevitably break?
Traditionally those letting houses owned the houses outright - the mortgage was paid off. "Buy-to-let" has changed this but perhaps that is not a good model.
So, in other words, they have a bunch of money invested in an asset that, if they sold it, could otherwise be earning some rate of return in some other investment.