> Landlords don't make money by providing a service or selling goods. They make money by sitting on something no one else is allowed to have.

Is that second sentence any less true of any other trade, including the traditional selling of goods? After all, the only reason you have to give a food vendor money for food is that the vendor (or someone on the vendor's behalf) will use violence if necessary to defend the vendor's exclusive claim on that food.

> Sure, they bought the land originally, but there's no more investment to be made after that.

That isn't true. There are maintenance costs and property tax, and less obvious things like the opportunity cost of risk of purchasing a house in order to rent it out.