IMO it's several inter-woven factors all colliding to create the mess that is TSA. (Pop psychology time! )

Americans are indoctrinated from birth to obey authority figures. They are culturally disposed to process solutions to the point that process becomes the solution, to the detriment of whatever the original goal might have been. (Think how many times you've heard "Those are the rules." in response to a situation that is outside the rules). Add in that TSA on the management side is made of people that are ex-US military and other government departments: they're not creative or (generally) intelligent people and take a militaristic POV (rather than security POV (& the number of times that Americans just respond blankly to that statement is scary)) to process and action. At the front end of TSA, you get a wide range of people that are frequently intellectually & emotionally unable to do the job required. While they're given (the wrong) processes and procedures to follow in a sort of faux McDonalds-fication of "security", they get no strong or even clear management direction or support in applying it all. As a result, you get a version of the "guards psychology" coming into play where the guards run wild. TSA reflexively defends them because it doesn't have the management structures or desire in place to monitor or correct mis-action, so it just feeds on itself.