"Qualified" person is not somebody who had evening school, and can hold screwdriver.

There is a chain of custody, paper trail, spare parts certification, logging... For example if there is a heat ex-changer that sometimes does not work, it may cause a lawsuit. McDonald (and insurance companies) wants to be able to sue ice cream machine manufacturer.

I also want to be able to replace any "suitable" parts in my car, but sadly government does not allow that! Remember, when everyone lost their shit with Volkswagen "cheating" in their software?