Company mechanics don't give a fuck that's why, either they are underpaid and/or underpressure to get as many vehicles done each night as possible. (Read up on how the mechanics trade works in general and its pretty shit, mechanics are often paid on minimum wage + "commission" on amount of work rushed through). It's not a field that people are going to stick their necks out and make themselves unhirable either.
In this case FedEx outsources the work to a fleet management services company to remove even more liability from themselves.