there is limited shelf space. they take the original product off the shelf.
You might argue that having generics on the shelves at all means that new small brands have to pay more for placement, but that's not the same thing as stealing product ideas and leaving no other option but the generic version of that thing on the shelves.
I've still never seen a brand new product show up in the store and become popular only to be pulled off the selves and replaced with only a generic version of that product. When I see examples of that happening, I'll accept that grocery stores are guilty of doing what amazon does.