Re: No, no store allows it
Not only that, but supermarkets will usually give preferential placement to their own brand "generic" products, making people who want the "real" product walk past the store brand. When a non-generic has prime location, like an aisle endcap or a display island on its own, they have paid for it. Prime placement is never given away free, even to major players like Coke or Budweiser.
If you invent a new category, like the keto products craze of late, the grocery stores chains are watching how many they sell and will put out their own competing products if the new category is successful. Often with the same flavors, ingredients, even the picture on the package may look similar. Look at the clone/generic products at Walmart, or Lowes, or any other major retail chain if you think it is just supermarkets.
People complain that Apple sees someone have success with an app like Spotify and then "copy" it with their own app (as if music streaming was some sort of amazing invention that should have been patentable or something) but such copying/theft is 100x more blatant in the brick and mortar retail industry. And the cut of the sales price they take is often a lot higher than 30%, too.
Granted, they don't continue to take a cut after sale (like "buying" movies to play on a smart TV) but if someone wanted shelf space in Walmart for a product that would be offered for free with money made after the sale none of which Walmart would see, they'd get laughed out of the place so loudly it would still be ringing their in great grandchildren's ears a century from now.
If "selling products on Apple's platform" is an Apple "monopoly" in some people's minds, then how "selling products inside of Walmart stores / on Walmart's web site" not also a Walmart "monopoly"? Should they be required to offer some space in their stores for third party stores that take less of a cut than Walmart does? Maybe Coca Cola wants to sell its products in Walmart with their own little display and checkout area so they can collect 100% of the revenue? That's basically what Epic is demanding by wanting to force Apple to allow alternate app stores.