As someone who lives in the US and reads every nutrition label to determine how much sugar / carbohydrate I'm taking in, my intuition is that it would be much harder in practice to do so if it were per 100 ml/g or by oz or something. Sure it would be easier to compare products, but the math would be come much harder to determine what you actually consumed, wouldn't it?

I mean, it's annoying that they do things like '3.5 servings per container', but at least you only have to multiply by 3.5 (I would just prefer a total on each container). If it were 'there are 5g of carbs/sugar per 100g of material,' it would end up being an extra step to determine total grams, divide it by a potentially wonky number or round and divide, then multiply the 5g times the resulting number. Sure it's not that much harder, but it would really add up throughout the day if you were tracking your diet. And IME that's a much more common use case than comparing nutritional value of different products by comparing the macronutrient makeup in grams from the labels, which I've actually never done once

Exactly. I don't understand why in the US it's also not standard to show nutritional values per OZ or something.