They aren't being subsidised. They're being paid to rent out their property.

And what would happen if the government passed a law saying that housing benefit could no longer be paid to private landlords? Looking over the instant wide spread homelessness, you would get a sharp spike in landlords either selling their houses or going bankrupt because their business model is no longer viable. Without that government money, they can’t pay their buy to let mortgages and the whole thing collapses - so they’re essentially getting property for free, paid for by the government, all because they managed to have enough capital to get in on the racket at the ground floor. Not to mention all the other less indirect ways the government passes laws to benefit landlords and property owners at the expense of renters.