It’s a common elemental theme we see across stories, fantasies, etc. It seems like it is an element that should be part of that pantheon but maybe there is a scientific explanation of why it isn’t? Or is it simply, they did not have enough understanding of it at the time those 4 were suggested.
How exactly ‘power’ of a bender is calculated? Like is it just training or you need to also have some kind of ‘understanding’ of the world and be spiritual enough?