If you walk into your favorite supermarket or grocer, you would immediately notice milk is most often sold in rectangular cartons. Right around the next aisle your favorite soda is sold in a variety of forms and sizes. Curved bottles and cans being the more popular forms.
Why isn't milk sold in cans or soda in cartons? Here is what I think.
Product design choices are made by manufacturers on a simple economic concept of Cost and Benefit.
The argument arises from the fact that for a given surface area, circular cans waste 20-30% of space compared to cartons. Which effective means for every 100 cartons I store in a warehouse or a shelf, I can only store 70 cans of the same total volume.
Warehousing, storage, transportation and store shelf spaces are expensive. Thus purely from a cost perspective, cartons are more effecient.
Soda is generally consumed right from the can or bottle. The most common consumption happens outside your home - on the move, picnics, parties etc. Producers know that the general consumer values convenience. The shape of the container makes for easier grip (bottles) or easier consumption and disposal (cans).
Thus, if the producer incurs additional cost in providing these product features but creates a better experience for his consumers, his product selection would be to maximize the consumer's value. This in turn would drive up sales and his profits.
Milk is generally consumed at home and generally poured into a glass, than consumed directly off the carton. Also, unlike soda, fresh milk needs to be refrigerated, which increases the total distribution cost. Thus milk suppliers choose the most space effecient design which minimizes cost without sacrificing the consumer's value.
With an increasing competition in the soda space, packaging also creates a strong brand recognition and indirectly facilitates consumer loyalty. Be it Coca Cola's distinct curved bottles or Pepsi's easy grip bottles. Cans, however have remained similar over time.