Just about any economic/financial transaction could be boiled down to one party "leveraging their capital advantage" over other people.
Someone providing a professional service is leveraging the advantage of their education/credentialing.
The basic assumption of trade and barter in history was that it was mutually beneficial. You provide me with something I need and I provide you with something you need. The issue of "leveraging capital advantage" doesn't even enter the picture until large disparities between people become the motivation behind the transaction.
The example you give isn't exploitative at all. Not everyone can be doctors - it wouldn't make sense for everyone in the population to go through that level of training because you don't need that many doctors. So one person goes through the work to become a doctor, and they provide that service in exchange for money. Meanwhile someone else becomes a plumber, and the doctor gives the plumber money for their service just like the plumber gave the doctor money for their service. Division of labor isn't necessarily exploitative, its pretty natural.
And most importantly, the doctor provides a service that benefits society. The plumber provides a service that benefits society. The car factory worker provides a service that benefits society. They all provide something concrete with their labor.
The difference between providing a service and profiting off a renter is that the the landlord does not give the good to the buyer because they know they can profit more from holding onto the good. Money flows from the poor person to the rich person
solely because the poor person is too poor to own outright. It not only is an example of the economic disparity in society, it increases this disparity, because the rich person keeps getting richer while the poor person loses a huge portion of their hard-earned income while not building wealth at all. And the landlord did not create the land, it was already there. In most cases the landlord does not even build the house, it was already there. The landlord simply uses their capital advantage to take possession of the land and the house, and then profits off of their ability to take that possession without adding anything substantial to the equation at all.
Like I said, if you look at the people who promote real estate investment, they constantly brag that "real rich people" make money without having to work. They say, "MAKE THE MONEY WORK FOR YOU SO YOU GET RICH IN YOUR SLEEP". Bullshyt, the money isn't doing any work, money is completely inert. What they are actually saying is they force poor people to work for them, and draw in the profits to themselves solely by taking advantage of the inefficiencies in our economy that allow the rich to leverage their capital advantage to exploit the poor.
Also many people CHOOSE to rent because it makes more sense financially. What is so wrong about providing that flexibility to them?
That would be just fine
if it was a choice. In Old Testament Israel, for example, it was legal to rent, but it was also required that every family be allowed to own their own property and that right could never be taken from a family. And it was considered a horrific sin and exploitative if a landlord began to buy up too much land such that others could no longer own their own or did anything to keep others from owning their own.
We're in a world where people can't afford to own their own home because so many rich people own 2-3-4-10 homes. In many cities something like 35-40% of dwellings are not owned by the resident. That is the fundamental driver behind high property costs and the lack of most families (especially black families) to be able to invest in their own future.