I suppose could have put an end to it and in most cases would have done us a favor but decided to let us continue on so he could prove the devil a liar and such .
Even if this was the case, and there was a real need to show everybody that humans cannot govern themselves without God, once you get to Noah's day, and Jevohvoh decides to kill 99.9% of all humans alive, what does this mean? Doesn't this clearly mean that by killing 99.9% of all humans that the question has been put to rest? This would have been the right time put this issue to bed, make the few humans left perfect, kill the devil, and let humans live happily every after.
Instead, Jeevava sadistically lets humans continue to be guinea pigs on his experiment, to show the devil that humans can't live lives without their daddy.