Do we trust the pilots not to unlock the door in that situation? "Open the door or the passengers die!"?
The reason why 9/11 was possible was because it had never happened. They could have added zero security measures, not even locking the cockpit door, and 9/11 still would probably not happen again. The reason is because before 9/11 the common wisdom was to let the hijackers do what they want. After 9/11 the common wisdom is the plane is going to be used to fly into a building.
But, locking the door is still a good common sense extra security measure.