I rather agree with Relia. I think that the Ghost Pokemon body is normally solid. But it seems like one of the defining characteristics of Ghost types is the ability to become intangible, aka pass through solid objects. It's like the DC comics character Martian Manhunter. He, like all Martians, are born with many abilities, one of them is the ability alter their normally solid bodies so they can pass through solid objects at will. And as you pointed out, Ghost types must use that power to protect themselves against the attacks of Fighting types and Normal types.