people don't in general do that
Oh, but they do. People are violent. You're old enough to remember that we had to be constantly reminded in mass media and by the stewardesses to not try to wrestle with hijackers; to do things the "civilized" way. People followed the rules until it was clear the rules didn't apply. Then they acted. And they've acted many times since. Sure, it wouldn't hurt to have a few more air marshals, but what good are four air marshals compared to a plane full of passengers with knives? Pen knives used to be carried by everyone everywhere. Now they're scary terrorist weapons.