Because having one plumber fix 10 houses is fundamentally different from having a landlord oversee fixing 10 things in the same house.
Imagine if every mechanic only fixed one part of the car and you had to go to 10 different ones to fix 10 different things. No mechanic would be able to point to what’s wrong with the whole car and can only tell you what’s wrong with each part.
There is a degree of vertical integration needed to maintain a single dwelling. As an example, I wanted to replace my stove that had a broken oven. In order to do so, I needed to fix the gas line. However, I need to finish removing an old gas furnace and installing a heat pump. In order to do that, I needed to repair the broken sewer lines under the unit, and in order to do that, I needed to resolve a dispute with the city over sewer line maintenance (they admitted fault eventually).
This wasn’t just a bunch of small projects that 10 people could each do one of. There were a myriad of dependencies and choices to make that would affect other parts of the house.
Funny enough, the same principle is part of why the US healthcare system is so shit because the lack of vertical integration due to the insurance system is why patients have such a hard time getting the diagnosis and medications they need. If you or a family member has multiple health issues, you may be familiar with this.
My point is, keeping a house alive isn’t some group project that you can get 10 people to each do a little bit of. At the end of the day there are executive decisions that will determine the outcome of other parts of the house.