Humanoids are part of nature, and as part of nature, they are natural. Nature has bestowed upon them not only energy to animate, but a soul. This soul is not present in divine, infernal or magical creatures, and therefore they CAN'T LEVEL UP. Experience is the refining of the soul, and with it people draw their inner strenght to their maximum limit. A creature that dies has its soul go back into the everstream, Earth itself, but it is for some time still bound to its mundane existence, and thus can be retrapped inside a new vessel.
What are souls? Souls are godly essence that grants to planets the ability to flourish life. Every being with a soul has some scale of godly powers, not really known to them. Were a great number of souls to be grouped inside a single vessel, this vessel could reach true godhood, and thus have infinite potential, and with that, infinite power
