While luggage is easily opened by cutting or breaking it, doing so is far more obvious. Being able to open and then re-close something without leaving any outward sign of the fact makes detection of it far harder, and therefore easier to get away with.
If half the bags going onto an airplane are visibly damaged, then red flags are going to be raised about it. But if no one notices that anything was wrong with their luggage till they get it home, well then only the people who have anything missing are actually going to report it, if they notice at all (Some may assume they simply forgot something and lost it on the trip, or not think about the fact that it is missing till well after the trip is over). Limited reporting then makes it harder to be sure where the theft actually happened.
Canon EOS 7D | EF 28 f/1.8 | EF 85 f/1.8 | EF 70-200 f/4L | EF-S 17-55 | Sigma 150-500