I worked at one of these places in college. Everybody had layers and layers of processes to make sure the shipment of controlled substances weren't tampered with. Because like you said the shipping companies were mostly immune to consequences from that. They'd wash their hands of the bad actors and move on. But it was seen was our fault if it happened and we didn't find it. Because of course we could be in on it and our not finding it could be part of the ploy!

In one a Fedex driver and conspirators would repackage a large shipment of painkillers. They'd take a single bottle from the a few boxes on each pallet and seal them back up in such a way that was very hard to spot.

Another time they'd take like 10% of the pills out of a bunch of bottles.

Many times the driver would get lunch and the truck would be stolen. Like once a year.