"It is difficult to get a man to understand something, when his salary depends upon his not understanding it!"

Nobody is going to put themselves out of a job by saying, "Ubiquitous security screening doesn't work, as every countermeasure we come up with has obvious counter-countermeasures. Thus, we should shrink this agency by a factor of five and refocus our efforts in other areas."

But they can't just ignore threats either, because that gets you hauled in front of Congress the next time something happens. Thus, they ride a line of carrying out countermeasures that don't really help, but which are enough so that they can go to Congress and say that they did everything they could.