So I'm guessing this body/clan training method is basically their gods "sort-later"/recycling bin.
Remember how bunch of reincarnation stories are basically "I woke up in the body of someone recently die"? This body just fill for the "lost soul", which might even be outworlder. Once they die in this body, however, they would be subject to this world law. So they die in training & another lost soul get suck in that muscle sponge. Repeat.