because madara does not belong to the specie "people" so he has no weakness.Kishimoto-sensei: "All people, even the strongest ones, have weaknesses, however, Madara does not." this quote came directly from Shonen Jump 35 interview of kishi and he is the writer therefore this is fact not theory :ice:
he is considered a tool thus he can be manipulated....my theory is he will be used to fight against tobi hahaha