They do sell half-gallons of milk in the gift shop, Fulton's General Store. I'm positive--we were just there in early August. I noticed, because I had purchased a half-gallon of milk at our last gas stop along the highway, and paid about the same! It was under $3, maybe $2.59 or $2.79. I was surprised to see what all they offered--bread, packaged breakfast baked goods, and snacks. Mostly convenience store prices, but still a great deal for use in the room. And there were a couple of bargains, including a $1.59 bag of cheese popcorn that my teens loved. $1.59! At WDW!
Maybe they sell out of milk, that could be why people don't see it. I would just ask when they get their supplies restocked. I found the CMs at this store to be among the most friendly and helpful anywhere!