Adam - unfortunately, rightly or wrongly, airside security is predominantely about stopping the bad guys getting in. It doesn't always work, but it does most of the time.
Unless you're clued up enough to notice yor loss immediately, the perpatraor of your theft will either be off shift, or have had time to pass on your goods to someone who is leaving before them.
It shouldn't have happened, but it does, and unless ground based intelligence is improved, it always will.