It's like you're asking:
" If the aurora borealis and ball lightning and sprites are are phenomena, what is phenomena made of. It has to be something. What is that something."
It's not like that at all. You're still missing the point. People are asserting something exists and haven't the faintest clue what it is.
Besides, I can tell you what both of those events are comprised of. Unless someone can tell me what spirit is, it's not even remotely the same.
From this one can see how Biblical and clergy use would have influenced the modern use of the word. As already mentioned, it has more than one meaning.
If God and Satan and angels and demons are spirit stuff, what is that stuff? So far all answers point to either metaphor or "heck if I know"