I am surprised it took so long for FEDEX to find out, since every package is tracked at every junction.
The drivers game the system, marking them as delivered and then taking them back, and similiar stunts. It seems easy enough to solve but FedEx doesn't take the effort until they lose contracts.
The executives at FedEx know they can't or won't pay for better employees or quality control and leave it up to the victims, both shippers and recipients, to sort it out.