Anyone they rate as a five-star player who doesn't pan out is a hit on their credibility, so the incentive is for them to limit the number of five-star ratings they give dramatically. Similarly, by stating the number of players they are going to designate as 4* and 3* players, with way more players as 3* than 4*, they are ensuring that an analysis of draftees will always show dramatic differences in the probability of being drafted from one star level to the next.

Here is a description of how ratings are calculated: http://www.cougcenter.com/wsu-football-recruiting/2013/2/5/3956800/rivals-scout-espn-247-star-rating-system-national-signing-day

This is the main reason I don't like articles like the one ribo linked to. That analysis of five-star players drafted vs. three-star players drafted is absolutely baked into the ratings system; it is basically impossible for the analysis to produce any other result.