Liters measure volume and kilograms measure mass/weight. 100 kgs of stone and 100 kgs of feathers weigh the same but there would be a lot more feathers. Whereas 4 liters of milk is the same volume as 4 liters of water.
Uniformity would be my guess to why they use volume measurements instead of weight when it comes to liquids. Not all the liquids weigh the same so you would have all different sized containers for everything.
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