The way I've always figured it, Cobra's chemical didn't simply make the Spirit seem dead, it also simulated a state of embalmment. In other words, he's 'dead', he's 'embalmed' - why not just bury him as is? It doesn't explain the lack of an autopsy, of course, but Central City's police force was notoriously lax about such things - quite often, the Spirit had to point out something that the forensics team had forgotten to check for.