because amazon owns the marketplace and has access enormous information on what sells well and can use for its own benefits. This creates unfair competition.
Walmart doesn't know where I live/lived, what I've purchased for the past decade, what I might be mildly interested in (searched for at one point), what movies I watch, what books I read, etc, etc.
Walmart cannot cater it's experience to what it thinks I might buy. Amazon can.
It's just not the same. There is a huge asymmetry of information (from both suppliers and customers) that makes Amazons overall practice feel much more predatory.
Oh, and Amazon is 50% of all online retail. Walmart is something like 10% of all brick and mortar retail.
