> if you need to lift that much weight, you'll be using a crane, in which case your strength is a moot point.

You don't typically lift all 20,000 pounds at the same time. The typical case is that you have twenty or so pallets with a thousand pounds on each of them and you use a pallet jack, which goes under the pallet and has wheels. That's how one person can move 20,000 pounds of freight at all. But you still have to overcome inertia and rolling resistance against 1000 pounds and have enough left in you to do it 19 more times. And do it fast enough that your truck leaves on schedule or makes it out soon enough to beat rush hour.

> Assuming strength does matter for loading/unloading, the real culprit is how truckers are paid, not the strength of the driver.

It isn't how they're paid. It's the fact that when a truck driver gets to a loading dock they're inherently standing around unable to go anywhere else until the freight is loaded or unloaded, so the efficient thing is to have them move the freight (whether you pay them separately for it or not).