Trent Dilfer had a historically good defense. Eli Manning was a very, very good player in his good years - have you forgotten his last name? Flacco also was playing great and was given the biggest contract ever given to a QB after his SB win - showing that he was a top notch QB.
The ones that are successful are outliers. I will die on this hill - if you don't have a top notch QB, you should be moving heaven and earth and cycling through them until you do.