Amazon ads are contrary to the company's mission statement of being the "most customer centric company." They could improve the customer experience at one stroke by eliminating pay-for-placement.
It is more complicated than that. Advertisers are customers too and advertising creates incentive to use Amazon and enter into larger deals with Amazon that ultimately lower prices for customers. Additionally, revenue from advertising can go directly into funding supply chain innovations that result in an even better customer experience. Amazon doesn’t typically do anything unless the total downstream impact is shown to be positive; while customers do get annoyed with ads, they appreciate the nice things ad presence provides for them to a greater extent on average.