This one is funny, and has a couple of obvious solutions that have been prevented due to internal politics. The short answer is that they have a couple of different recommender systems, all competing against each other internally for sales lift.
One is purely based off of pageviews. When you get recommendations for something you already bought, many times it is because you looked at it, but they don't know nor care if you already bought it. In their words, it works really well and accounting for sales brings in a lot of needless complexity.
Another is based off of sales. They also don't care if you already bought it because according to them, it works well. I remember trying to point out to them that for some types of products (specifically consumable products) this would work really well, but durables not so much. They claimed otherwise, that although they couldn't explain it, it was entirely common for people to rebuy things like vacuum cleaners and TVs and kitchen knives. I did a tiny bit of research to show them why they thought that, and proved with a small segment (vacuum cleaners, I believe), that after you filter for returns and replacements, that the probability of sequentially buying two of the same vacuum cleaner was effectively zero. They asked me to do it for the rest of their products, but I didn't have limitless time to spend on helping another team, especially one with a PM who was a complete dick to me for having the audacity to make a suggestion that he hadn't thought of.
In all, I believe there are a dozen or so recommender services, each with their own widget. There are tons of people that think all of the recommenders have merits in some areas and drawbacks in others, and the customer would be better off if they merged concepts into a single recommender system. But they all compete for sales lift, they all think their system is better than the other systems, and they refuse to merge concepts or incorporate outside ideas because they all believe they are fundamentally superior to the other recommenders. Just a small anecdotal glimpse at the hilariously counterproductive internal politics at Amazon.