Worse, the way TSA tries to use manpower as a way to justify its existence creates mind-numbing conditions for the screeners. Imagine day-in and day-out having to look at thousands of people who are not happy to see you, who are late for their flights or are just in a lousy mood. Now you get to rifle through their personal belongings (something even a sworn police officer couldn't do without probable-cause) because the supposed high-tech scanner just returned another false positive.
TSA is not seen by passengers as helping to make them safe, the screeners are seen as a nuisance causing the passenger to be late. Unfortunately, having a large number of screeners touching passengers inevitably leads to this;
"Former TSA Agent: Groping Scandal Is Business as Usual"
The pat-down is the default setting when all else fails. If a scanner is malfunctioning or down, then pat downs are the only recourse. Even though TSA utilizes agents training to note behaviors indicative of a suspected terrorist, the fall back position of "better safe than sorry" forces many more passengers through pat downs then is necessary.
At the end of the day, the question is how many potential terrorist attacks has TSA stopped? The answer is the majority of TSA doesn't even know. TSA screeners receive no feedback whatsoever on their interdiction efforts so they continue to go to work without any idea on whether or not they have stopped the next 9/11.