The problem is that one is ASSUMING "Ghosts" exist as per definition "a Spirit of the dead". without ANYTHING to back THAT theory up. GHOSTS are a proposed EXPLANATION for what the unexplained phenomena we may CALL "Ghosts"
In other words, we DO have EVPs, we DO have consistent witness reports and Photographs of Misty Humanoid forms, we DO have photos and witnesses to Human, often in historical clothing, that appear and disappear. It is NOT that these phenomena do not exist or that they are invalid------They are often REAL phenomena that our Science cannot explain YET. ------ The problem is, we do NOT KNOW what is going on. Calling them "SPIRITS OF THE DEAD" or "GHOSTS" is only a groundless PROPOSITION based on a superstition.
This does not mean the "evidence" is non existent, but evidence for WHAT? Logic tells us that there will be phenomena and events that we cannot explain, simply because we or our Science has not explained EVERYTHING in our Universe or nature YET. In These cases we make observations and measurements, report them, and leave them "open-ended" until we have enough data to propose a THEORY.--Its how Science works.
You do not IMMEDIATELY deem an unexplained event or phenomenon as being some sort of a "mythological" figure or caused by one. ("WOW! Nobody knows what THAT is--it must be a GHOST!") Also you do not dismiss the observations of evidence out of hand either ("He called that phenomena a GHOST, since Ghosts do not exist, nether did the phenomena")----Which is just as illogical and un Scientific.
So the question stands, Just become SOMETHING is real, and we don't know what it is, WHY would it necessarily actually BE a "Ghost"? We are getting tangled up in our logic here. What the hell is "evidence" for a speculative "answer"? "That "glowing human shaped mist" is only evidence for "a glowing human shaped mist"------Its NOT evidence for a "Ghost" and it dies NOT become "invalid" or non-existent" simply because someone CALLS it something that may not be existent (Like a Ghost)