FedEx wants the truck to look good. I mean nothing falling off of it or damage. They assume because of the age, there has to be something wrong with it - their company image matters and that's OK.
Landstar - same thing but age doesn't seem to matter to them, their work is their brand.
They all look for leaking boxes, damaged doors, punctured walls, no securement points and bad floors in the box. They also frown on busted up fenders and crinkled sheet metal.