It's not the exact same job if it's not the exact same commitment of time between the two employees for starters. The physical demands in the 8th and 9th hour of these jobs take a far bigger toll on the body over the years than the person working only 3 to 5 hours.
In any event, I'm a full time driver having to navigate far greater risks than anything a loader inside the building would ever have to account for. A loader doesn't have to evade loose dogs, put up with ORION, deal with black ice conditions on asphalt after the sun has gone down, try to explain to immigrants that they can't pay for a COD with pennies, nickels, or a credit card...the list goes on.
If you want the similar wage, you most often have to make the similar commitment. It's always been that way there.