Possibly because most shipping involves trucks with either separate cabs and cargo areas os shipping containers.
They can turn the X-rays off and on you know - scan the cargo area, but not where the drivers are.
On the few occasions where I have carried my camera case (with film inside) through airport custom inspection, I have 'politely requested' that they have 'allow' me to take my still 'sealed and unopened' boxes of film out for a 'visual examination'. My request has never been 'denied'
I believe it is because I made the request' 'politely' rather than aggressively denying the x-ray scan.