The landlord is effectively a lender, or at least a rusk-taking intermediary. They buy the house with outright cash or a mortgage backed by good credit. Their risk is that they don't find tenants, can't receive rent, or the house price goes down.

It's effectively shielding the tenant from the financial risks of house ownership, for a price.

I also agree it's unfair to the poor that renting is more expensive long-term when they can't afford to buy, but I don't think it's the landlords fault.