> They don't stop hijackings (locking the cockpit door does that)

9/11 also stopped all future hijackings. Up to that point passengers were trained that if they stayed calm they would likely survive. Now? Short of the hijackers getting guns on the plane, passengers will absolutely fight back.

> they don't stop bombings (there are much better targets for that, which don't involve killing the bomber)

Suicide bombers are probably the main vector that TSA helps avoid even if they miss some items sometimes.