Even yourself called it security theater because people have looked at TSA security and never found a single case where it prevented a threat to a airplane.
To quote: "Security theater is the practice of investing in countermeasures intended to provide the feeling of improved security while doing little or nothing to actually achieve it"
Security theater do not cause 90% prevention. And to be clear, I know several personal cases where a knife has gone through the security check because the person carrying it had forgotten that it was even there. Its not effective, no research say its effective, and all benefits that security checks provide against threat to airplanes are the placebo effect.
Strong doors, investigators, and bomb sniffing dogs. Those are some real security measures found to be effective on airports, airplanes, as well as other places where dangerous people need to be stopped. They are used by both police and the military, and shown to be cost effective. The methods used by airport screening is used by airports, not by the police or the military, and the results are clear.