This is the point that is always missed when talking about "security theater".
Smart and practiced people can make anything look easy. An attack on an airplane is simply too challenging for the vast majority of people to execute. Is it impossible? No, but almost nothing is impossible.
Making an attack difficult on a plane also eliminates the majority of severally mentally unstable from carrying it out. You can kill 300+ people in one instant, it is impossible to stop the truly motivated and capable (ultra tail risk) attacker, but it is crucial to stop the rest of the tail from casually taking down an airliner.