Weight repartition and balance is extremely important in an aircraft. If everyone moved to one end of the aircraft it could cause loss of control and crash
a change in weight distribution is cited as one of the reasons for the crash of this us cargo flight in afghanistan.
https://en.wikipedia.org/wiki/1981_Pushkin_Tu-104_crash happens more often than people know.
It’s quite scary to think that you can crash an airplane by having everyone walk to one end
Why break the cockpit doors when you can just get a bunch of terrorists strategically running around the plane to manipulate it just at the right angle of attack to steer it towards a building?
Simple solution is to pack enough people in that they can’t move like that