1. Because milk needs sterilization, Tetra Pak packaging is often used. This tetra pak packaging is a composite material of paper, polyethylene and aluminum foil, similar to the properties of paper. The best structure of paper is square. Tetra Pak didn't make round paper bottles until May of 201/year, which is exclusive in the world, that is to say, it was impossible to make large-scale mass-produced round bottles with paper before that.
In addition to technical reasons, cost is also a factor to be considered. Because milk needs to be stored in cold storage, it needs a freezer, which costs more than ordinary shelves, while square saves more space than round and can store more products.