What also bears repeating: hardened cockpit doors and policies to resist hijackers, while completely effective against a 9/11 repeat, are passive. Unless there's an actual incident, people don't see any action being taken and "Durnit! We need to do everything we can to keep the terr'ists from repeating 9/11!".

So you get the TSA, which is the "actually doing something" layer of security.

Even the most ardent pro-TSAers will admit it:

You: The TSA fails over half of undercover tests, so it's obvious that if someone wants to get a gun or knife onto an airplane, they will. Given this fact, how do you explain that there has not been a single successful aircraft hijacking or bombing since 9/11?

Pro-TSAer: You have to layer security. If a terrorist manages to make it through the TSA checkpoint layer, the cockpit doors and policies to resist hijackers are the last line of defense.

You: But given the fact that terrorists have a greater than 50% chance of getting their guns and knives through the checkpoint, it's not even a remotely effective layer!

Pro-TSAer: I'm not talking about this.