During a recent Amazon all-hands meeting, the company leadership talked about how there are something like 5,000 employees working on various marketplace frauds, including using ML on some of this.
I have no idea why it's so hard, but I don't think Amazon is being as complacent as it seems from the outside.
Frauds where Amazon loses money, or frauds where Amazons customers do not get the product of its purported reputation, or maybe not even the product they thought they ordered.
They were specifically talking about marketplace fraud, where Amazon customers are misled but no money is lost. Customer fraud (against Amazon) is a different thing, though there's a bit of overlap when it comes to fraudulent returns.
I have no idea why it's so hard, but I don't think Amazon is being as complacent as it seems from the outside.