1) It's hard to hide things in a physical store. There are some tricks but if you want Brand A cereals and the store has them, you can find them in the cereal section. Amazon has in my experience, sometimes hidden product brands entirely or beyond page 4-5, favoring their own or prime products.
2) When a supermarket takes on Brand A cereal, the Brand A owner makes little risk, they get paid whether they sell or not. The supermarket is taking all the risk. If they bring their own store brand, they still run the entire risk, now for both stocks. If cereals stop selling well (because there is a milk shortage) then the supermarket is loosing a lot of money. Amazon faces no risk, they can have the vendor for Brand A continue to sell non-prime cereal from their own stock so amazon faces 0 costs if it flops. If the vendor does well they can be onboarded for inventory management and prime shipping, amazon takes more risks and more profit. Lastly, they can use methods like "We suspect the product is counterfeit, send bills for when you bought it" to gain direct access to the manufacturer of Brand A. Now they can sell Brand Amazon directly from the manufacturer, leaving the Brand owner of Brand A in the dust by selling the same product, that did well, cheaper and hiding them in the search results below their own.
The supermarket equivalent would be that the supermarket subsidizes the Brand A seller from their value-add in-store things like customer payback cards after a while, then blackmails them into giving them the manufacturer by threatening to not sell their popular product, then selling the manufacturer's product under an in-store brand and the Brand A cereal is only available under-the-counter. Brand A seller is also entirely co-dependent on the supermarket as that supermarket is 90% of the market.
I know you have wallmart in the US but in the EU we do have some variety in supermarket chains (ALDI Süd/Nord, LIDL, Edeka, Rewe, etc.).