Because only a 5 year old is going to make all of the Bills a 99. I'll usually tweak some ratings a bit, mostly on the Bills (mind you it takes a good amount of time to edit them), and also make some rookies lower, bump some players up, etc.
The real problem with madden is that people (and players) get mad with "low" ratings, so everyone gets a high rating to keep them happy. I think the "average" rating for all NFLers is an 80 or 81, which is absurd. If madden wants to rate players, they should be an average around 75 with a standard deviation of 10. This way, only the good players are highly rated, and only the real good players are above a 95. But that would hurt too many feelings, which is why you end up ike last year's madden where there were something liek 6-7 WRs rated a 99.