Everyone knows a pilot can easily kamikaze an aircraft, including CASA.
If pilots could waltz through security and get airside without being scanned, that is a very obvious hole in the system that could easily be exploited. Either the pilot carries the weapon knowingly through security before passing it on an accomplice who was scanned already and came up clean (thus allowing an atrocity to take place without having to kill oneself), or some enterprising terrorists sneak the necessary pieces into the flight bag etc and get them out again once airside - distract and switch with a similar roller bag, whatever.
I mean it's far fetched but not that far fetched.