@goodhunter You might have missed this info. The whole point of leveling is for the MC to own a body that can withstand the power of shadow monarch. If you rewind the story back where jinwoo get the blackheart the architect is confused by the fact that jinwoo remain the same and not taken by the real one.
As to why there is a need to find and train a human to be the vessel of shadow monarch? i cant tell you that. Can the other monarch leveled up? They are the top of the foodchain, each of them have their own mastery, they are not bound by a system, its them that able to make system in the first place. So for such a being, leveling is out of the question.
The thing with MC here is out of the usual and the other monarch are still their own self and about ruler, the story will tell about it better than i can. enjoy the ride