The more complex you make a system - any system - the less reliable it becomes and the more expensive it becomes.

As TSA adds more and more to it's screening, and as it seeks to keep true American citizens in the dark (said citizens being educated would be more aware of something that might be out of place), the more risk it adds to the system. And the less likely that the TSA will find mistakes it makes itself.

It is plausible that the reason that TSA keeps process secret is that it doesn't want to have someone expose shortcomings of the system, thereby scaring the public, having the public propose a better way, or raising demands to reduce the agency. NIH.