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.
Well, static electricity has been well known and well documented since ancient times… that is about the same time as people started talking about fundamental elements.
I guess they just didn’t consider it important enough.
(And yeah, they didn’t know lightning is the same thing.)