There's a pretty long list. If someone sells a fraudulent product by mail, I don't hold the mail carrier accountable. If I buy a product I don't like on Craigslist, I don't hold them accountable.
The mail carrier does not advertise the fraudulent product and does not handle the financial transaction. There is a pretty obvious difference between a mail carrier and Amazon. Craigslist actually is responsible for stuff that is offered on their website, see also the long-running debates about sex workers advertising on Craigslist.
It is not only the customer who suffers damage, but also the seller of legitimate merchandise who is not compensated by Amazon for fraudulent sales. Also, Amazon 100% has a spreadsheet somewhere calculating the share of customers who don't bother claiming fraud compensation.