Yeah, but you were already scared of contravening the rules and wouldn’t have cheated anyway.

It’s a false equivalency.

Different students have different tolerances for risk. Anti-cheating measures dissuade some but not others. Students with a lot of risk tolerance and a very high drive for success at all costs will likely find ways to cheat no matter what, but that doesn't mean simple anti-cheat measures don't work to reduce cheating.

In my experience as a TA, cheaters are more than willing to do so even with a proctor standing right next to them staring at them.

If anything, I suspect that on net a noisily invasive anti-cheating system would actually increase cheating, as more students would be tempted to see if they could game the cheating system than would be dissuaded from cheating from hearing about a cheat-proof system.

Obviously the cheaters you would catch while standing over them are the cheaters who would cheat when you're standing over them. Selection bias.

I can tell you with certainty that some cheaters are skittish. I am certain of that because I once wrote a bunch of trig identities on the side of my leg. I was very nervous about it, and only did it because I knew from previous tests in that course that I could get away with it. If the teacher had been walking around during that test, my pant leg would have stayed down.

I agree, but it is not just a matter of risk tolerance, as the emotions desperation and/or entitlement are factors.