^^ I have a theory...
Water is essaintially H2O... Hydrogen and Oxygen is always available in air..and the chemical structure of water is not that hard..so I guess they reform water by allready available resources, whereas food's chemical structure is very complex... lots of vitamins, proteens..you know
So food is hard to create...
Magic is a kind of science. Even muggles can produce reform water from air..but they cant do the same for food.