Because I have severe thantophobia, and things like guro and necro help to lessen the fear a bit.
I'm taking a concept that terrifies me on a fundamental level, and turning it into something that gives me sexual gratification. Its wasn't something that I set out to do, but its something I've come to gradually understand after stopping to think about it for a bit.

Other than that, I also have a domination fetish and do dom IRL, so its essentially an extreme version of that; complete and utter control over another, even in death.

That said, it's something I could never bring myself to do in real life though. A fantasy is a fantasy, but I could never to that to another real person, even if all that's left is technically dead cells and tissue. I just can't take the final summation of a person, and reduce it to literal fuck-meat. I'll fantasize or roleplay it, sure, but like rape, its not something I could ever do or wish upon another person.