Even if Amazon knows and organizes the fact that you've bought the thing, that isn't what the ad space is selling. They're selling space that targets a particul

Even if Amazon knows and organizes the fact that you've bought the thing, that isn't what the ad space is selling. They're selling space that targets a particular kind of person, and whether or not you've bought the thing doesn't matter. Like, the company buying the advertisement slot wants to target a particular sort of person, and this is measured in various ways. So the network selling the advertisement slot wants to group visitors into different buckets based on these measurements, and sell to whoever happens to pay the most for a bucket that view happens to land on. Basically, it's the end result of a process that confuses a measure with what it's measuring. You want to optimize for "interested in buying X". What you can actually look at is "things that people interested in buying X do more often than people who aren't interested". People buying advertising optimize for that, since it's actually possible to do so. And then since buyers actually care about the measurable proxies, that's what ad-space sellers give.