This one again!
Apart, maybe (big maybe) from your home base, Security won't know you from Adam. If a person could pass through simply by being in a pilot's uniform and flashing an airport pass, who is to say that they didn't steal it or even forge it? They have now passed through security unchecked with a loaded weapon. Duck into the loo, change back into civies and board their intended flight and target.
I have a NATO security clearance and I'm not exempt from airport security. As a passenger I'd prefer that to extend to crew and all airport staff too.