"How hard is to remember to unload your weapon before packing it?" I guess there's no I.Q. check for firearms purchases, maybe there should be.
IQ and attention to detail are different things.
Also: Even the best-trained, most reliable, gun user can have a lapse when in a hurry, as in when packing for a flight.
That's why firearms training stresses redundancy, with rules like "A gun is loaded as soon as you put it down and look away". Or "Don't point (even an "unloaded") gun at anything you don't want to destroy."
The phenomenon is referred to as "a visit from the Ammo Fairy". That entity is similar to the Tooth Fairy, but instead of leaving a coin under you pillow it leaves a round in your chamber. B-)