Earlier quoted context omitted.
Grocery retail is much more complicated. They may charge shelf space rent.
They take on the vast majority of the inventory risk, which is the fundamental detail everyone seems to miss. They own what they sell. The complications come from trying to manage that risk - manufacturers who have better products or marketing can afford to pay for prime shelf space which also means it'll move faster.
It's still not the same as signing up to creating your own store on Amazon though.