Being the giant chain is the optimization. Grocery stores have razor thin margins; operating at scale is the only way to keep prices reasonable.

This is also one of the few domains where people are very price sensitive, and they regularly see and think about costs. I couldn't tell you what my local automotive guy charges for an oil change, but I can absolutely tell you the price of flour and cheese.

No one is going to pay $10 for a single apple, and if you raise prices hard enough people are going to start gardening.