Calorie density is not something made up. You are redefining a very well-known term. Many legumes, grains, and root vegetables are made up of copious amounts of water in their prepared form.
Calorie density is also not the only metric for recommendation. Everyone agrees that liquid calories are not "felt" by the body in the same way as solid foods.
By your standards, Coca-Cola is actually less calorie dense than boiled potato, but I don't think anyone would recommend a Coca-Cola diet.