I don't see the contradiction. Janitorial duties are at the very worst easy to train any person off the street for. As long as people need any sort of minimum wage to survive you can find a janitor.

But that also means that, because minimum wage, your salary will almost never exceed the value brought to that business. Outside of some super crazy regulations of cleanliness.

Not necessarily true. You either pay the margin for a contractor, or pay the janitor more in-house.

Janitor is a messy and low prestige job. As such some will choose a lower paying job if they get the option and the pay isn't too much worse