A simpler explanation might be "They don't know you bought the thing." Either because they're different retailers, or because the data are hard to put through f

A simpler explanation might be "They don't know you bought the thing." Either because they're different retailers, or because the data are hard to put through from the checkout to the marketing department. Maybe it's hard to connect Google's idea of who a user is with the store's user accounts. Maybe the ad-buying algorithm is suuper simple, and getting a dev to "do it better" isn't worth the cost. If it's just "this person searched for car rentals two days ago, let's show them car rentals today," that seems simple enough to make money over "no targeting," with obvious and acceptable failure cases like "already booked a car" and "phone got stolen, should show ads for pawn shops instead."