Considering that Amazon could have run a simple account search to determine that I am definitely not interested in seeing ads for a product that I already own, I think that the AI-targeting ad revolution is a while off yet.
That is unrelated to Google, but I haven't seen anything there that would suggest they are running anything smarter.
The deal is those ads show up when the ad space is the cheapest. It's known as remant inventory. So the retargeters can pick it up for very cheap.
The deal is the companies KNOW that some people see ads for things they already bought. But some people are seeing ads for things like only considered, and bombarding them with ads does affect behavior. The question is, is the increase in revenue worth the cost of retargeting.
The answer is very much yes. Even as it annoys people who already bought it, which doesn't even factor since they already made the revenue from your purchase.
A lot of assumptions we make don't hold up when you start gathering real data.