Okay, inspired by that Australia vs Canada thread, and a recent ATRL thread about milk bags. It seems that many people here think that it's weird, and why is that? We don't actually drink the milk out out the bag like that, instead we put it in a pitcher and cut a small hole at the top of it. If anything it's more convenient than the cartons/containers.
We had them back in my country and we liked them because they were cheaper (8 pesos for a liter of bagged milk whereas the milk in carton cost 11 pesos, but then again the milk in carton had a nice picture of a cow).
The US tends to sell most milk in this large gallons which is good because selling more helps save money specially for families. I guess that's why milk bags aren't an American thing.