[Reincarnate]

Because we generally eat meat. Our poop is full of pathogens. Cows don't have the same problem because they're largely grazing animals that eat lots of plants. You can use composted human feces, though, as long as you're properly removing all the heavy metals and stuff like that.

But no we don't exactly see a nearby Humanburger joint where we can partake in burgers made from the meat of the dead -- largely for health and moral reasons.