“Even if you found a CY gallon, it wouldn’t cost any less than 2 half gallon bottles.”
Under normal rules of retailing, this is not true. Just look at the chalav stam prices. Also, look at the difference in pricing between quarts and half gallons. The unit pricing (per ounce, per quart, per pound, per piece) is always less when you buy larger quantity.
I suspect it is because the milk company, distributor and retailer can make more money selling two half gallons than selling a single gallon. Given the large size of families that consume chalav yisrael milk, kein yirbu, and the quantity of milk purchased, it is a lot of money for them to leave on the table.