It was the job I have been most satisfied with in my working life. However, I did only do it for 8 months for 6 hours a day. Maybe in the long run it gets more boring.
The variety between shelf filling between different store brands and even different stores of the same brand made me really think no two jobs are the same.
Not my field, but perhaps you need to widen your view a little.
I'll bet there was someone at your store who was the Shiva of Stacking too?
I only worked checkouts (pre barcode scanning, other stores had it).
That being said there are a lot of jobs where I have difficulty imagining what mastery looks like; say in an automated, low choice style field like bus driving. I couldn't recognise an expert bus driver from a relative novice. But that probably only reflects my lack of knowledge about bus driving.
But a lot of it is just for show. Not really that impact-full on the work done.
People do it because they get a sense of achievement out of it.
But isn't the psychological sense of achievement suppose to be a reward for completing meaningful goals ? Isn't this "empty calories" in a sense ?
Look up "Bus Rodeo". It's basically a skills challenge for bus drivers.
You'll see drivers make maneuvers with 60 foot articulated beasts that the average person would have trouble doing in a normal SUV.
I'm sure you could measure it financially, and that a nicely done shelf stock makes more money than a poorly done one.
Depends whether someone gets to just fill the shelf as per plan or has the curiosity to ask "WHY", noticing why X things fly off the shelf why Y things two shelves lower stay there forever. This curiosity builds the critical thinking 'muscle' and can motivate a shelf-filler to switch into marketing (hypothetical scenario has to do with a 17yo working part time and then deciding to studio marketing)(I didn't study marketing) :)