After you are locked into their ecosystem, they prevent you from selling for less elsewhere, even on your own site, they coerce you into paying them ad fees to boost your listing in their search results, and if you don't pay them enough they will put your competitors above you and redirect searches that are specifically for your product to other similar items.
Then, if despite all of that you do well, they will find some company in China to make an "Amazon Basics" version of whatever it is that you're selling and then undercut you on price and boost their listings above yours.
You might even say that this is a win for consumers since they get fast cheap access to products, but the product quality is always a race to the bottom and because this happens so often it results in the death of product innovation from good companies because they have to spend so much brainpower and energy keeping the Amazon machine happy while it consumes them.
Ebay is a little better, but that is also rapidly enshittifying as well.