I agree that not everything that could be done, has been done. However, I feel there are plenty of good reasons for not doing absolutely everything to make airports 100% secure. You can never have 100% security, 100% control, and 100% speed in any operation. Where you opt for more control, you take less security and speed, more security, less control and speed.
We have to keep a focus of the primary use of airports: business travelers. Time is money in the business world, and a quick way to curb economic growth in this country is to make people wait in line for four and five hours while every last person and every last bag is searched. I doubt that our the TSA's actual mission is to stop every last suspicious item from getting through security. Their mission is, most likely, to make the risk determinant great enough that the reward is miniscule in comparison, therefore deterring terrorists from using airports to carry out their attacks or causing them to resort to more sophisticated and involved planning. As I have stated earlier, the more sophisticated and involved a plan is, the greater the odds of our intel operatives picking up on it. Now, I doubt you will find any stated mission of TSA that states they are not trying to find every last lighter, toe nail clipper, and loose wire in all luggage that passes through checkpoints, just as every Battallion CO will state that they are committed to bringing every soldier in their battallion home at the end of the war. Realistically, they know that they will have achieved success at bringing home 90% of the battallion and having met all their objectives.