Ghost and Shadow

Why do all ghosts seem to look like the sheet ghost. It doesn't even matter what you looked like before you died. Hell, does it even matter if you're a person. Maybe in the 'verse everything turns into that sorta ghost. 
That chicken certainly did. Now his true form is just a shadow behind the ghost! 

Now, as to what 'verse I'm talking about. I couldn't say. Certainly not the world of "Dual Chicken" or "Shades McCoolguy"... Though maybe "HERO"

Obviously I didn't think this through.