There are others in the this comment section saying it is easy to take a gun through TSA. If that is true, why would an attacker want to bring another box cutter? If the TSA is ineffective, couldn't someone bring a tool to break the cockpit door?

My point is that none of these things have to be impenetrable. There is a swiss-cheese model to safety. TSA doesn't have to be 100% effective to be effective. Every layer of security adds up. There is no single component responsible for all airline security, not even the cockpit doors.