If routes change, and It needs a larger package car, UPS is not going to check to see who has a DOT card as to what size package car to put on that route.
Or, Bob called in sick today and his route is hanging. Joe is the only available driver, but he can't run that route because he doesn't have a DOT card in order to drive that P10.
Or, Bill broke down on route. The only available spare package car is a P10, but they can't send it out to Bill because he does not have a DOT card and cannot drive that package car.
It's easier to just require all package car drivers to have a DOT card.