That's not how they work. What you're seeing is the issue with not having data shared between networks and the resulting poor targeting.

Retargeting has better results than any other type of targeting but unfortunately there's a gap between the ecommerce systems knowing you bought something and the ad networks used to market those ads. It's easy to drop a pixel and say you looked at a product, but it's much harder to let that same network know that you're no longer interested for whatever reason.

Add in complexities like multiple devices, browsers, ecommerce stores, payment methods, and it only gets harder. This is usually why retargeting is much better with smaller direct-to-consumer companies which tend to have a single customer database and payment method to sync from.