Because 99.9% of customers are stupid, and the other .01% are chomping at their heels.
Customer - "Hey you, where's polenta!"
Manager - "I'm sorry, sir, but due to manufacturing issues, that item is temporarily out of stock."
Customer - "well, go look in back!"
Manager - "I'm sorry, sir, but we don't have any."
Customer - "Ain't you even gonna go look!!"
Manager - "Sir, we do not have ...."
Customer quickly stomps off in a rage, finds another clerk, and asks them the same question. Upon getting the same answer, they angrily demand to see the manager.
Then there's the customer who, the day before Thanksgiving when the store was packed with people, lunged over the counter and assaulted a deli clerk because they didn't like having to wait for their order to be prepared.
I spent nearly 30 years dealing with customers. I ended up having a complete mental and physical breakdown.