Economic Conundrum: Square vs. Round Containers
Update: Thanks for all of the comments to this question, I agree with "ty b" who said:
A: "Milk requires expensive refrigeration, so the shippers and sellers want to maximize the use of refrigerated space. The square containers do that."
That is the answer I had in mind. Milk and cream require constant refrigeration from the time of processing and packaging, during shipping, and at the point of sale, and square containers maximize the use of refrigerated space. Water, beer and soft drinks don't need constant refrigeration, and so square packaging isn't an issue. Another example of square containers is the Tropicana varieties of pasteurized "Pure, Premium" orange juice, which would also require constant refrigeration.