No mythology, contrivances, or manipulation are necessary. Even from a completely secular perspective, some form of rebirth is arguably more likely than simply non-existence after death. Things like karma are unnecessary. If rebirth is real, even one driven entirely by naturalistic processes, then the state of the world is enough to make us ask what kind of world we want to be born into next. The more suffering there is in the world, the more likely it is you will be reborn as something that will suffer.