Ads don't solely exist to convince you to buy a product. Some ads are designed to make you feel good about the purchase you already made. or to teach people who will never buy a product to respect people who do.
In a completely different context, ads can be used to reenforce status. That was a separate distinct point gp was making. If you want to buy luxury goods, but can't afford them, a well placed gucci ad can act as a brand anchor & keep you yearning.
Your point stands, that the "mega retarget post amazon ads" are a flaw in the system, but as the system gets more data, they will be tweaked to be more effective.
This specific case - "here are some other vacuums you didn't buy" - is one that seems more likely to hurt my brand loyalty and post-purchase feelings than anything else.
Chances being that anything that irks you about your new vacuum are going to be most accutely felt in the initial period.
So reminding you heavily that there are other, greener-grass vacuums out there seems productive in inceptioning "I'll never buy X brand again!" resentment.
They can do that with random data and it'll work just as well. It's called divination and you don't need to track me for it.