It's to keep you afraid and compliant.  As I'm sure you're already aware...

Not to mention they want the TSA to be wasteful and ineffectual, so that economic conservatives can argue for privatizing it so they can leech more taxpayer money through contracts, or so that they can do away with it altogether and return us to the pre-9/11 days of airline profit motive driving down security coverage.

Eternal vigilance is the price of liberty.  Sure sucks about that eternal part.