The pilots were probably killed at early stage, then the planes were flied by terrorists.
There is a reason why all planes were Boeing 757-767... these terroists were probably trained to flight this model.
Why not Airbus?
The pilots were heros, they could and probably did resist.
I don't believe only knives were used, I think something more lethal was involved so the crews were killed/controlled in such short time. Eg, that flight taking off Washington.