Nonetheless, I hope you recognise that incompetent idiots beget more incompetent idiots, if they think they'll get away with it. You don't want e.g. a spate of bank robberies, by idiots who've heard that rubbing lemon juice on your face makes you invisible to cameras. It doesn't matter that they'll get obviously get caught, the problem is a spate of idiots attempting bank robberies (because they're filled with confidence they'll succeed) could easily get people killed.

I don't like security theatre either, and clearly the whole thing is a job creation program and an excuse for vendors to sell flashy scanner devices. But you need visible deterrents, even if most people know they're theatre.

They also act as reassurance for idiots who wouldn't fly otherwise. Idiots' money spends just as well as clever people's money, and there's a lot more idiots out there than clever people.

Because we live in a society with a free press, we have the chattering classes asking "what can we do about this threat?", and government is expected to respond. People don't like to hear from the politician "you're idiots, we don't need that, you are no less safe if we do nothing", they like to hear "we're doing XYZ to address this threat, how clever and wonderful you all are, dear citizens, for recognising it. Your safety is my top priority", then we get the https://en.wikipedia.org/wiki/Politician%27s_syllogism