Go ask DHL/Airborne Express why it flopped with the contractor model. Then go ask UPS why it works so well without it.
UPS doesn't have that gap between operations management and the drivers. UPS doesn't have contractors whose agenda is to make as much money as they can and whose obligation is to meet the minimum expectations of the contract with Ground (or DHL, or Airborne Express). With the costs incurred and the revenues that can be lost in the time sensitive package business, the operation has to have control of those trucks and the people driving them.
UPS gets incredible productivity from it's workers and runs a ruthlessly efficient operation. For decades, they kept their old, surplus vehicles around for Peak, so even in the 90's, it wasn't uncommon to see a package car from the 1950's on-road. The basic body lasted for what, 30 years? And the basic chassis was a Ford or Chev with a straight-six and manual transmission. Simple, but effective.
They also lead in technology. The roll-out customer for the then-new 767 freighter was UPS, and in typical UPS fashion, it was a no-frills airplane, built for profit. Same with their tractors. Basic, but fuel-efficient and kept until the wheels came off.
They retain skilled, productive workers because they pay excellent wages and have a great retirement and benefits package. The average UPS driver has a stop count that would be the envy of any FedEx manager, and UPS routes are generally very tight, adding to the efficiency factor.
In short, they have control over every aspect of their operation, and their leadership actually knows how to run the organization. In spite of high labor costs, their business model works quite well.