I'm finally going to ask someone, why is the milk always in the back?

Because it's closest to the coldest parts of the store

When designing a grocery store your attempt to isolate the hottest sections like the deli from the coldest sections like for anything that has to be chilled and cold or frozen so when they build the store they put all the compressors for cooling stuff as far away from the the ovens and stuff for the deli so they don't fight each other and waste a lot of electricity it's just a coincidence that the milk is usually at the back of the store.