I think part of the reason that's not currently done is that it would mean ratings would be recalculated a lot more frequently. Either every time you viewed a download (e.g. looking at the list of the 30 most recent downloads) its rating would have to be recalculated in case the helpfulness of any of the reviewers had changed, or else every time a user's helpfulness changed, the rating of every download they had ever reviewed would have to be recalculated, which in some cases number in the hundreds.