Walmart has all of the sales data on every single product they sell. And anyone who wants to come to Walmart has to provide their sales data.
Or you have Target that is movely heavily into generics right now. Nearly 1/3 of a Target store is stocked with their own brands.
Retail is harsh and it is not unique to Amazon at all. The only difference is that Amazon has infinitely wide isles, and they host a swap meet outside the doors.
Here we have: Tesco, Asda, Morrisons, Sainsbury's, Aldi, Lidl, Waitrose, M&S.
In the UK nearly all towns have at-least 2 supermarkets, and most towns have another 3 within a 20 minute cycle/drive.
The fundamental problem with Amazon is as with Standard Oil. They have no competition, and they have no separation between their logistics/warehouse business & their marketplace. Thus, they are using their delivery business and warehouse arm (which in itself would not be sustainable) to ward-off competition. Just as when Standard Oil bought the railway companies.
Walmart is far bigger than the grocery stores you have listed. In fact, the grocery store part of it is relatively new. Can you buy tires or tents or bikes at a Lidl?
There are tons of regional grocery stores in the US. Each region has more different regional brands than national brands.