Have you bothered to ask yourself where the money goes after it's put in the pockets of Bezos' and other Amazon investors? Do you think that money is just sitting in the bank doing nothing?
Last time I heard Amazon is struggling with net positive cash flow because it keeps on re-investing profits into various other side projects with eventual goal of dominating retail space e.g. Amazon Go.
Amazon is in the business of creating a marketplace that efficiently matches buyers and sellers. It provides a distribution infrastructure for sellers and provides a high level of service for buyers.