>>508763409
You're forgetting the power of the Shinigami Eyes. Light should have used that damn book to transform himself into one, since they're immortal. A god of death can extend his lifespan using the notebook, but humans can't do that. In fact, this is the main purpose of the shinigami using the Death Note. It was revealed by Ryuk that many gods of death have died because they were too lazy to write in their notebooks. Their use of the notebook to keep them alive implies that they not only depend on them, probably originated as humans or other spiritual beings who knew how to use the Death Note correctly.