I had the misfortune of working at a supermarket earlier this year. Needs must. Anyway, the pickers had a hard time picking their orders at times, because shelf stacking was often not as efficient as it could have been. Plus, the supermarkets have a habit of moving products around to keep shoppers on their toes. Additionally, each store has a different layout, so would need a bespoke version of the app. Supermarkets also have a vested interest in having people wandering around their stores.
On a related first-world problem, supermarkets operate so cheaply* because they are staffed by armies of people paid marginally over minimum wage, often toiling under stressful conditions. I did 12 weeks and it was horrendous.
* To them.