If it’s really impossible to miss,
Apparently it is possible to miss. There have been numerable studies on this phenomena and it boils down to the way the brain is wired. We see what we are conditioned to see, and block out the noise. Most TSA agents behind the x-ray scanner screen will never see a prohibited item pass through despite the thousands of hours watching hundreds of thousands pieces of luggage pass through, so when it does appear for that unlucky agent, he/she is unlikely to register seeing it.
Why do you say most TSA agents will never see a prohibited item?
I fly a lot, and it’s quite common to see people taken aside and have their bags opened.