Why do you think that? Just curious. That's one of the easier jobs in the building for me, second only to driving irreg trains around. The way I see it, literally the worst part about loading is constantly having to climb ladders to break jams and such. Outside of that, it's not that bad.
Sorting causes all kinds of joint problems because, even if you go out of your way to avoid twisting, it still ends up happening little-by-little, without you ever noticing, and just a few months of it will wear you down.
Unloading has you constantly on the ground lifting heavy packages onto the belt, and while the methods help some, it can get old really fast.
Loading is just standing there, moving stuff from the extendo or the rollers, to the space directly in front of you. If you know how to build a wall and manage the space in the trailer when stuff starts spilling off, then you're set.
I can do most jobs in the building at this point (I passed up the opportunity to become Hazmat certified...for now), and loading being the most physical doesn't reflect my experience at all. I'm just surprised to hear people say that it's the most physical job around. I still say that's unload.