Many Amazon sellers only sell on Amazon, or have a large majority of their business sales there. This is equivalent to having insight to almost your entire business.
Most brands at Walmart and other stores are sold many other places.
There are many businesses who are 100% dependent on Amazon as a platform and Google as an acquisition stream. If you want to break out and sell on your own online platform, you're still dependent on Google, be it through search or advertising.
I would love to hear of consumer facing (B2C) online businesses who are successfully operating without any Google or Amazon dependencies to see if it's even possible in the current online ecosystem.
My hypothesis is that it isn't, and as such Amazon and Google should be broken up. They have close to a functional monopoly on consumers, but I'm putting forward that they also have a functional monopoly on online businesses in commerce.
Which goes back to the buyer problem, that they don't consider other channels. Then you get hostile co-development of browser extensions for cross-channel price comparison, and life in the jungle continues on as such...
Most brands at Walmart and other stores are sold many other places.