The difference between Walmart and Amazon, is that Amazon can study products it had no analog for. It could decide that it wants to enter a market, study sales patterns, force a supply chain change that let them spy on it, and launch their own products from that data.

The depth of data they have is much deeper. Walmart hasn't had 3rd party sellers for as long as Amazon, and that sort of historical sales data is priceless to retailers. They don't willingly share that.