I am not from the US but I think Amazon has much more market dominance than Costco. If Amazon had 20% market share nobody would mind, but they don't have real competition that comes close.
Amazon has less than 20% market share in retail overall. Remember their competitors are not just the online sellers who are pushing this antitrust angle. It’s Walmart and Target and grocery chains and CVS and Walgreens and brick and mortar department stores and so on.