Because they don't know if you bought it or not. They are gambling that you didn't and are trying to capture a sale for the ad buyer. Presumably that is somewhat effective since it's so common.
As shit as showing you ads for the thing you just bought is, it may be the best ROI they can manage. Which is pretty damning for the whole enterprise.
Obviously the result looks stupid to us as consumers which is quite memorable. I suggest it is a by-product of intentional choices. IMO this is a better explanation than "oh this company is just too stupid to do it right, but also they are all knowing about everything you do and are just so doggone evil." Maybe they are that evil, but I am sure they have learned how not to overturn their apple cart and this is an obvious result of that.
"We will always automatically exclude recent past purchasers of your promoted products."
So there's some notion of this data being available, at least internally.