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.
Electricity is pretty much just very directed fire if you think about it in a sorta abstract way.
Thats why ATLA was objectively correct that fire benders could also handle lightning.