[_dark_matter_]

Why would they? Because they are about to do the thing they planned to do for months or years? Because they may be risking their own life? Because they're worried about getting caught rather than following through? Because no matter how prepared they are they have never done that EXACT scenario before at that exact airport with those exact people? Because the human mind is a lizard brain even with training and preparation?

Still not a perfect systems, other countries manage this part much better (I've heard Israel is especially good at it, but I don't have direct evidence).