People think that they're in the land of the honest once through airport security and thus lower their guard by a few percentage points.
I was queueing at the gate in Glasgow some years ago, and looked down by my side to see my black trolleybag gone. I looked across and saw a chap walking off with it. I ran over and said that was my bag to which he apologised for walking off with the wrong bag! He had villain and liar written all over him.
They purchase a £10 easyJet ticket, get themselves a boarding card and then prey airside.