>If I'm a third party seller on Amazon, why should I / what do I gain by investing into the optimization of my listing if I do not depend on my Amazon listing for the majority of my revenue?
Sure, you're right, that's how businesses think, and it makes sense. But the side effect is the situation that was brought up, that customers don't understand why their favorite products don't show up higher up in the list, if at all. there might be a point where it can affect a brand and their domain authority. Like Google search, the stuff that doesn't show up on the first pages is perceived as less relevant.
Sorry for this late response, just saw your reply.