There's a few reasons. Oftentimes the milk cooker is refilled/restocked from a walk-in cooler behind it accessible only to employees, and it would obviously be

There's a few reasons. Oftentimes the milk cooker is refilled/restocked from a walk-in cooler behind it accessible only to employees, and it would obviously be difficult to put this anywhere other than the back wall. I suspect it's also easier to run the utilities for freezers and coolers around the perimeter of the store and then down a few aisles.