Because if you're a landlord as an individual, a a human being, you're not what people mean when they say "landlord". You rent property - you can do that with a conscience, but that doesn't deserve the title of landlord
The term "landlord" refers to people who own homes as a business - people who create layers between them and the people they affect, bureaucracies or sheer numbers they can min-max without guilt.
That subtle difference is everything
Context matters - the person I rent from is my landlord, but that person is not primarily defined as a landlord. They rent out a couple properties, but they have a job - being a landlord is not their career
You can call them a landlord (and they can call themselves one in certain contexts), but in the larger systematic context someone who rents out a room obviously is categorically different.
The line is blurry, but honestly I don't think it matters if you rent out your basement, your old house, or even a few houses. At some point it becomes a full time job (for someone), and that's where I think the line is
And as far as companies, the landlords are the ones who own the company holding ownership.
It can also refer to the company itself as it's a person legally (unfortunately). It's not used that way in everyday conversation
But in everyday conversation it's normal to refer to the manager of the management company as your landlord, which is often an employee of a company that oversees bookkeeping and maintenance hired by the actual owners
Ultimately, I think it's important to fight for this distinction because language changes with use. By dragging in everyone who owns a second property or rents a room, we draw a line on the wrong side of working class people and their family who aren't the problem