https://nypost.com/2021/10/19/i-found-an-amazon-folder-with-...
Had him 99% there. Payback is great on these users.
If a few really did buy already? Who cares, beats showing the ad to a room of 20,000 random people
It's because it creates a new profitable business model for BigTech.
Personalized ads require that the network collect as much data about you as possible. The idea is to use this data by trying to create some behaviour model to determine how and when a user purchases stuff. (Currently this doesn't really work as advertised, as you guessed).
But the US government, with PRISM, created a new revenue stream for BigTech - buying the raw data of the users directly. As with any government contract, this is actually way more profitable than serving ads to cheapskate internet users. Seeing how easily and successfully BigTech is able to get this data, the US has effectively decided to privatise spying (partially) to the BigTech. To sabotage the creation of regulation and privacy laws that may prevent such data harvesting, and to create an even more profitable market for the BigTech, they have also started to rope in other countries too. (See Five Eyes (FVEY) - https://en.wikipedia.org/wiki/Five_Eyes ).
The alternative business model is so simple and is already in place --- simply advertise based on expressed interest.
This is what DuckDuckGo is doing very effectively. Show ads based on search words. No global, privacy invading network required. Amazon is doing the same quite effectively. Their ad business is growing by leaps and bounds simply by showing ads based on search terms.