Why not ghosts?
Because they aren't even coherently definable. Like, a Sasquatch claim we can do something with. There's zero evidence, but at least there's a testable claim: a hitherto undiscovered family (in the biological sense) of large primates are dwelling in the forests near where I live. Presumably we could find one. I've yet to see a quanitifiable definition of ghosts, so where would we even begin to look (graveyards?), and what would we be looking for (spooky stuff?)?
Let's look at the definition provided by the OP, as it looks pretty standard: "ghost (n)., a disembodied soul; especially the soul of a dead person believed to be an inhabitant of the unseen world or to appear to the living in bodily likeness."
Okay, so a ghost is a "disembodied soul." Which is a...what, exactly? The definition is already completely circular, and thus an absurd claim on its face (literally reductio ad absurdum). And that's without even bothering with mumbo jumbo like "the unseen world" and various weasel words like "appear to be" and "believed to be."