You really shouldn't try to analyze what these people are doing any more than you can wish for the moon. As a couple of others have alluded to, airport security staff have one purpose ony - that is to provide the Government of (*insert your country here) with tangible evidence to the voting public that they are doing "something" about terrorism.
The Government doesn't care that it isn't effective or remotely useful, they only have to show that they are doing "something". Look at it from the Government's viewpoint;
1. Secures votes for demonstrating they are fighting terrorism.
2. Gives thousands of jobs to unskilled workers.
3. Keeps the educationally subnormal off the streets.
4. Brings the aircrew down to earth by treating them just like Joe Public.
5. Allows them to introduce random and arbitrary changes to the screening system to maintain the illusion of responding to terrorism based intelligence.
6. Generates additional revenue from usury based pricing in airport shops for all those things you can no longer bring with you.
7. Provides a perfect scapegoat if something does go wrong and something gets on board a plane.
8. Has single handedly revitalized the sock industry.
9. Provides a pre-screeing selection program for fascist bully boys for when the Government decides to use them to maintain civil order when democracy breaks down.
Who knew?