Not at all.
The most common sold measure of milk is the gallon. And that's a handy quantity in both size and weight to carry in one hand.
If you were to go to a different measuring system, say 1 dex (equivalent to 10 gallons) you'd still have to ship it in 1/10th dex containers.
Because that's the size that humans use.
It's not arbitrary at all. It's about being useful for everyday human activities.
