When someone rents that apartment and they pay the landlord rent, the landlord is making money without working. The landlord isn't actually providing any value or service, they're only refraining from using the state to remove the person living in their property because that person is paying them.
If a person were to occupy the property without that landlord's consent or awareness, it would cost the landlord literally €0 barring damage to the property. If you factor in damage, the "fair" price of rent would be nonzero but negligible, in the order of several hundred euro a year.
Is there any reason why this landlord ought to receive payment for providing 0 value? Let alone enough payment to fully finance the property over the course of a couple of decades, during which time the tenants are effectively paying for the landlord's mortgage without receiving any stake themselves.