Because Amazon can arbitrarily raise the prices of goods sold at Amazon, and have no concern about being undercut. For example, you are selling a Widget for $2

Because Amazon can arbitrarily raise the prices of goods sold at Amazon, and have no concern about being undercut. For example, you are selling a Widget for $20. Amazon says "we'll take $10 of that ourselves". You can sell your Widget elsewhere at lower overhead, but you can't pass those savings on to the buyer, which means the buyer has no incentive to shop elsewhere, which means there are no market forces encouraging Amazon to reduce their cut. There are also no market forces preventing Amazon from increasing their cut. Tomorrow, they'll want $15 of that $20.