What really changed in those 40 years? He is still doing the same job, with the same effectiveness. It's not really unexpected that the $/hr would remain the same. Honestly I am more surprised that it correlates that well to inflation.
Off the top of my head: lower rates of unionization.
See also:
* https://en.wikipedia.org/wiki/Motor_Carrier_Act_of_1980#Crit...