Because people eat by servings, not by fixed numbers of calories. Comparing by semi-arbitrary servings isn’t perfect, but it’s better than not comparing by servings at all, and you haven’t offered any serving sizes that you believe are better, so semi-arbitrary is the best we have.
Servings are fine for candy bars, but they’re almost totally meaningless if we’re talking about fungible ingredients like spinach; those are going to be used in all sorts of ways, almost all of them different from whatever the relevant regulatory body had in mind. (Milk and eggs are a bit less so since they’re often consumed in quanta of one egg or a glass of milk, but neither one’s exactly an uncommon ingredient.)
I’m not sure there’s a perfect way of comparing nutrient density under these circumstances, but volume is probably what I’d go for; you can only fit so much on a plate, so ingredients generally displace each other on a volume basis. For leafy greens in particular I might use cooked volume, since they usually cook way down.
That doesn’t mean that people don’t eat by servings, it means that 30 grams isn’t a good serving size.
Furthermore, since we’re comparing different foods, the fact that 30 grams may be too small is compensated for by the fact that the serving size for milk is a cup, which is also too small.
Because people eat by servings, not by fixed numbers of calories. Comparing by semi-arbitrary servings isn’t perfect, but it’s better than not comparing by servings at all, and you haven’t offered any serving sizes that you believe are better, so semi-arbitrary is the best we have.
Servings are fine for candy bars, but they’re almost totally meaningless if we’re talking about fungible ingredients like spinach; those are going to be used in all sorts of ways, almost all of them different from whatever the relevant regulatory body had in mind. (Milk and eggs are a bit less so since they’re often consumed in quanta of one egg or a glass of milk, but neither one’s exactly an uncommon ingredient.)
I’m not sure there’s a perfect way of comparing nutrient density under these circumstances, but volume is probably what I’d go for; you can only fit so much on a plate, so ingredients generally displace each other on a volume basis. For leafy greens in particular I might use cooked volume, since they usually cook way down.
Who eats 30 grams of spinach and then stops?
That doesn’t mean that people don’t eat by servings, it means that 30 grams isn’t a good serving size.
Furthermore, since we’re comparing different foods, the fact that 30 grams may be too small is compensated for by the fact that the serving size for milk is a cup, which is also too small.