I stock shelves every now and then, and it's hard work alright, but so is cashiering. And cashiering is all about upper body work, while stocking involves the whole body. Also, cashiering means doing a lot of things at the same time, being nice to customers, handling lots of cash, working a computer, learning lots of produce codes by heart... yes, I understand why male cashiers are so rare.
There are a few jobs out there that require a physical strength that most women don't have, but none of them are in a grocery store. Really. It's not like we're digging trenches or something.