Asklemmy

Popular beliefs influences people's beliefs, which reinforces popular beliefs. Step back even farther from the question for a moment and ask, "why do you think of ghosts as dead human spirits at all?" That a "ghost" is some sort of dead human spirit is a concept that has been built into Western society for a long time. It is something we just accept in story telling and mythological belief systems because it's been in them so long and is told to us via authoritative figures in our lives from an early age. To tell a story where a ghost is anything other than a dead human spirit or the echo of a dead human, makes people call bullshit on the story, because the story has broken a long standing societal expectation. Sure, some stories can get away with it, and more so in the modern age where we are starting to appreciate stories which subvert long standing expectations. But, we still tend to fall back on old tropes and devices which we can expect readers to understand, without having to spend too much time on building a world. It's far easier to save the term "ghost" for something much like a dead human spirit and just create a new term when trying to describe something else.