In contrast, google a question about bags of milk and you get a heap of people trying to answer this conundrum--why use an inferior technology? Here are some examples: here, "so we drink milk in bags, does that make us weird?" [yes, yes it does], wiki and here.
To give bags of milk a fair shake, here:
I do love how this video shows how complicated this milk delivery system is: need to cut the bag just right, need to keep the big bag's clip with expiration date (which is stamped on plastic jugs), and so on.
Of course, the real superiority is that one can simply open a gallon jug of milk and pour. If you have bought bags of milk, you need to get a container, make sure it is clean, open the bag without spilling anything (no crying necessary with a gallon jug), pour it into the container and then pour some of the contents into one's bowl of cereal (like this bit of Hawaiian hotel holiday excessiveness):
The arguments about the milk bags are more efficient argument might be instructive, except that in Canada, the dairy industry is so highly protected that it can charge quite a lot and is relatively immune from the market--supply management or mismanagement as I like to call it. So cry me a river of milk tears if you think that the market up here is efficient in any way, including in the use of bags.