For fast food, the marginal value of an hour of work is a measure of how much a business can make from labor and the position, not some innate quality of the person. It’s flipping burgers not rocket science.
All companies pay people as little as they can to keep a certain amount of employees of certain quality around to do the work. The fewer options you have (or the more options your employer has), the worse the deal you'll have to accept becomes, and the lower your pay will be.
As for skills, I know plenty of people in IT who would go crazy working retail or interacting with customers within a month. Flipping burgers may be the easy part, but resilience against customer behaviour and monotonous/uninteresting work isn't something everyone has.
It is cognitively simple for you, because you aren't thick. But for people of well-below average intelligence, flipping burgers and doing something else at the time is just not possible.