In airports you have airside and landside. Aka non secure and secure. Either you are in secure area or not. You can check in lagal guns, knives and all kinds of things that is not allowed in the cabin in youre checked in baggage. They are checked in outside the secure area, and must be picked up otside as well.
The custom area are secure in a way they will stop you trying to get back in, but this is because you should not be allowed to hand over items to other people. The door to this area is not a secure one way door. And you could easyliy try to go in this area if you really try, but police or custom agents will make you turn back. Try to get inside secure area this way.
I have seen some airport where the bag area is inside a room, like OSL, but its ony one big sliding door separating it from the arrival area. Nut sure its a one way door, as bag claim service desk is in the same area.
But the main reason, is you cant have people accessing their checked in bags in secure areas.