Because 90% of my time with Madden is spent in franchise mode.
I love the gameplay, but when there are seasons when several rushers top 2000 yards (something that has been done 5 times in NFL history I think) and multiple players have 20 sacks (again, something that I think has been done 4 times in NFL history) it ruins the authenticity of the franchise.
If I wanted a fantasy franchise, I'd play with a fantasy draft like I did with Madden 08 and draft Peyton, Moss, TO, Gates and Bush and throw for 9,000 yards in a season.