Most of the "oldbiz" buy the goods and sell them from their own corporation -- this is because they have actual retail stores. Amazon does not.
I do not know how much their rake is for 3rd party online goods though -- I see "Sold by Pharmapacks", etc.. often on Walmart and the price is usually not competitive at all compared to "Sold by Walmart.com" items. Mostly has to do with the shipping price.
That's because Walmart doesn't offer third-party fulfillment (to my knowledge). They allow select third-parties onto the selling part of the platform (but again, not open like Amazon), but fulfillment remains the 3rd party's problem to handle, which is why the shipping price is still an issue there.
Amazon is providing far more of the value chain than other e-commerce sites, and it's no surprise to me that they're charging for that. If that doesn't work for a given seller, they shouldn't use Amazon. If enough people don't use Amazon, Amazon will have to adjust. I see no evidence that the latter is occurring in great numbers.