Because all your Perf calcs are then invalid
Of course they are invalid and that is just the point. By simply rotating without regard to how the aircraft responds, you are asking it to do something it simply cannot do. By gradually rotating, you will discover that it isn't ready to fly and can keep the nose down until you get enough speed. The Spanair pilots pulled the nose up too far for the
actual condition of the flaps/slats. It's no surprise that it didn't fly but if they had eased the nose up and got no correct response, they might have been able to finesse it.