Actually, if you look at stats, he's rated pretty much where he should be historically. For example, of guys who played more than 3 years, he's 22nd in pass rating with the likes of Carson Palmer. Which is about right for him.
He's 27th in passing TDs. He's 21st in passing yards.
So I am not sure which stats you are looking at that tell you he's one of the best in the game. The stats say he was an above average QB lumped in with the likes of guys like Carson Palmer, etc. That's pretty accurate if you ask me.