I speculate that this is because the role of driver is a high-trust and critical operations role. This person has to reliably execute package delivery every single day regardless of the highly variable problem spaces that emerge. Without the last X mile capacity that a driver brings, all of the infra at UPS doesn’t provide value. For example a driver wakes up early every day, rain or shine (or sleet), and is accountable for making sure a truck is loaded correctly at the dock, and every package on the route is delivered. This can include packages of diamonds insured for 10’s of thousands, to hundred plus pound boxes.
I only rode shotgun for a few weeks a year and it was hard work. From that POV it a seems like a fair wage for the role.