US Dairy companies have a huge lobby in the milk industry sector. Couple that with their contracts with plastic companies, bam, you have them advocating the use of milk cartons and such because it gives them the market dominance. Then the push of these in media etc, makes the move from plastic containers to plastic bags so much harder for Americans to grasp onto. Just like it took the US a long time to move away from the use of glass to cartons now. It's a nasty industry eh, only way is if the government pushed control over it which is not coming anytime soon.
But yes bagged milk is better for the environment. It uses less plastic compared to the bottles/cartons etc. Generally people pour it into a container anyway