The explanation I have heard previously is that people have bought an item are actually more likely to buy another: unhappy with the current purchase, or people who need multiple items. Sure, for most people it’s irrelevant as they are happy with their purchase, but most ads are ignored anyway.
One of the other things to consider* is that the details of person X that is looking to buy item Y are a snaphot at the time that information was brought/sold.
When person X buys item Y from company Z, company Z is unlikely to broadcast that fact. Ergo, the 'system' doesn't know that you have now brought item Y and that you no longer have an active interest in purchasing Y.
*Things may have changed in 20 years so the above is just an outdated observation based on a brief stint at a turn of the century B2B marketing start-up.
This is partially the answer. Sometimes people are running ad blockers that block sending events to Facebook, so the "Item Purchased" event never gets sent, but the account was created with an email that can be used in custom audiences, so the email is retargeted as if they never purchased the thing.
Or someone goofed and forgot to include the exclude "item purchased" event in the criteria
at the same time it's very easy to run into anecdotes people have like "you bought the 5th edition of this college textbook? do you want to buy the 3rd edition in Spanish?"
