It's probably because stock checking is a simple repetitive task that humans tend to fail at due to boredom and mental fatigue while robots would accel at it because of the fact that it's simple, well defined and boring.
I dunno if people expect there to be this huge warehouse of stock "out the back" of every supermarket with loads of shelves holding tins of beans or loaves of bread or whatever just sitting there. The shop floor was the "back room"!
I used to just tell folks that what they see is everything we had, so if they cant see it then we dont have it.
Even with fancy modelling and "just-in-time" delivery stock-outs still happen - I remember one year working on the last weekend before Christmas. We "sold out" of turkeys - in reality it was just that people had them in their trolleys and hadn't checked-out yet so people were stealing them out of other customer's trolleys when they weren't looking! Happy Xmas! :-)