I suspect like with most jobs, customs workers do the bare minimum. The workers don’t really care about the drugs because they see it so often it’s normalized.
They let stuff through eventually(hopefully), but packages take a long time to pass because they have a quota of how many they will check per day. Otherwise the system will just sent them more packages, and it becomes never ending. If they do less, the system will see that they are full and won’t send more packages to check.
That being said, it is really bad when so many packages are stuck, I suspect the algorithm caught onto some kind of pattern PCT24x7 is exhibiting.