[alphazard]

Product quality is difficult to measure. Even if it was easy to measure (as you seem to claim), it's difficult to know if a improvement in metrics is attributable to the product manager or some other factor. There are a lot of forces driving product evolution.

Contrast that to metrics around demand generation. If you have a sales team, they can work "how did you hear about us" into the conversation. Maybe you are tracking where redirects to the website come from. It's much easier for a marketer to prove that 100 prospects got in touch as a result of something they did, than for a product manager to prove the product is better because of them.