Several potential reasons:
1. Name recognition. I am sure there are reviewers who have that, "Oh its Madden, it must be good" attitude.
2. Many reviewers probably don't really know or care about sports games or they may be "causual" Madden gamers.
3. EA is so powerful that some gaming sites may feel intimiadated to give that good score.
4. They don't play the game long enough before reviewing the game thus not finding the bugs or gameplay issues that people in this forum will.
5. Who knows what goes on behind closed doors with gaming sites and the big and powerful EA. Is there a "you scratch my back and I'll scratch yours" attitude? I don't know if that is the case but one really has to wonder from time to time.