Every hour spent manually reviewing product pages could instead be spent growing food, producing shoes, building homes, providing accounting services, etc.
This doesn't mean it is or needs to be determined centrally - just that it is an economic truth that will bear out regardless. Amazon might hire 1,000 workers for this, pulling them implicitly from all of the other jobs they would otherwise have taken.
Maybe it would be a net-win, and maybe not. But from society's standpoint it really doesn't matter whether it is profitable for Amazon or not. We'd be trading less fraud on Amazon for less of everything else.