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.
Yeah like another comment said there is evidence of air benders collecting moisture from a cloud so I don’t really know. Then again maybe he was water bending? He did know how to water bend by that episode.