They sometime are and sometimes not. The milk that I buy is not in a cylindrical container. So the mathematical [logical] analysis of the question is that the premise is false and so the question is not valid.
Having said that, a sphere is the most efficient shape in the sense that a spherical container required the smallest amount of container material (glass or plastic) for the largest volume of the liquid. However, a spherical container is not stable, and the liquid could spill out. The best stable shape is a cylinder.
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Volume is a pace which can be occupied by some thing, whereas capacity is applies to liquids as the quantity contained in a given space.
For storing large numbers of containers I think a rectangular shape is more efficient because you can place them side by side with no wasted space between them like you would have with cylinders. I think it depends on how big the containers are (cylinders are easier to roll) and how much space you have, what type of liquid is to be stored etc. as to which shape would be best.
Cylindrical shapes offer structural stability and efficiency in many applications. Their uniform shape distributes stress and weight evenly, making them sturdy and resistant to deformation. Cylinders also have a large volume-to-surface area ratio, which can be advantageous for containing or transporting liquids and gases. Additionally, cylindrical objects are often easier to manufacture and assemble compared to more complex shapes.
The amount of anything a container can hold is it's volume or its capacity.
The volume of liquid in a cylinder is directly proportional to its height. And the height can easily be scaled and etched onto the side of a cylinder. and fluid containers creates statics voltage as fluid moves inside of it. so there should not have corners ( it reseves static voltage) spark can create through the corners.