That's because HP is only at it's max for a normal vehicle engine near max or redline RPM's. Before an egines ever gets to max HP capability the tranny etc usually upshifts to drop the RPM and thus lowering the HP output. Once the egine has reached max RPM, it's torque and HP output drops off fast and you can't gain anymore speed and usually will begin to lose speed. So you or the tranny has to upshift anyway or stay at the engine's max HP output. Not much good to have 500 or 600 HP if it's hardly usable over a reasonable RPM range! It's torque that turns the drive wheels and moves the load!
OTR trucks have many more gears to choose from which is a big help but it still takes the torque to turn the drive wheels or NO HP will be produced at all!
Much is even more true in gas engines than in diesels. In a pickup with a diesel, max RPM or redline is around 3000 RPM which is about half of what a gas engine's is. 1500 to 1700 is already max torque in a diesel engine pickup but max torque isn't acheived in most towing gas engines until 4500 to 5500 RPM's. Have to rev a average gas engine to over 4000 RPM to have any reasonable power output for grade climbing or for accelerating to join vehicles on a highway.
High RPM's suck more fuel as each cylinder has to have adequate fuel in the compression mix for an explosion the take place and 2 time the explosions per second means 2 times the fuel consumed on average. Then add in the known factor that diesel fuel already contains 11%-13% more BTU's per equal measure and also not having the inefficiency loss thru high RPM's followed by gearing down to drop the high RPM down the drivewheel turn RPM.
