What benefit might human review have? Maybe they could make sure the SERP list entries actually have the keywords you're looking for. Even better, they could make sure the prices in the shopping section are correct! Maybe even make sure they relate to the product you actually searched for... I might actually pay money for that.
Only some things scale like that. Google's insistence to use the same model everywhere has gained them a deserved reputation as having atrocious support.
That's how you get economics of scale.
Google couldn't have a human in the loop to review every page of search results before handing them out in response to queries.