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.
I think it was lumped into fire. Since lightening does cause fire in nature. Moreover, I think fire stood for heat as well since fire is pretty much the only way to naturally heat something