Because that has a non-zero chance of customers exiting out of the sale. I’m guessing they measured the lost sales against fraudulent affiliates and chose fraud affiliates.

People are funny and the slightest distraction might skunk the sale. Not for most, but for a few percent.

I watched my grandmother purchase something online and it was revealing how much time it took and how much she struggled. Something that took 10 seconds for me took her 15 minutes. Amazon wants those sales too.

It would be better to pose the question after the sale is complete, imo. Ask the costumer which partner, from a list, contributed to get him/her to decide for the product.