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.
You’re getting into the problems with the magic system of the ATLA universe when taken to its extremes.
If you really let them all go hog wild people wouldn’t be fighting in a combat sense but more in a instantly kill you kinda way. I can see why they stopped at lightning for a kids show. The blood bending was already a bit extreme and I am still a bit surprised Nickelodeon let that happen.