Having already purchased an item is indeed highly correlated with purchasing it again, however, you are ignoring the temporal element. Yes, I will need to buy a new gaming monitor for my son, but hopefully not for another three years, and by then most of these ads will have disappeared.
But for all they know perhaps you want to purchase one for your daughter now or a second one for a dual-monitor setup. Ads in general have a very low success rate, so it can be worth doing even if the guess is wrong a very large percent of the time
