Long story short, logistics benefits from scale. The cost to ship a pallet of poptarts to a store is roughly the same as shipping half a pallet. Smaller stores can’t really undercut larger corporations because they don’t have the scale.

Also when your mom and pop store doesn’t make a profit for 3 months, they go under. When a Walmart doesn’t make a profit for 3 months, they stay open because they have those loses spread across hundreds of other Walmarts.