Labour/scale, indeed. The facilities where these things are usually produced have the capacity to make sure they are sterile. Hospitals cannot sterilise items to the same scale.

You just need to be careless once for it to be a problem. And considering how many sterile items you need to use every day - particularly in an emergency - it is not unlikely that one item went back into the reuse cycle without being sterilised. It's to avoid the question, 'was this item sterilised?' If its wrapper is open, assume it is used. You can only do that safely with single-use items.