when in RPG when have much more possibilities, also room on arbitrary, because existing rules of creation are much more fluent
No. Because fashion, sociology, culture, religion, linguistics, anthropology, history and historical circumstance, media and all the things that led to the creation of these settings are no less constraining than the laws of physics. Which are also not absent from these settings either. You can create a world where magic or technology or metaphysics are such that the laws of physics do not apply such as we would expect them to. But there are obvious limitations to that, as creators will not entertain every nook and cranny of the implications of their toying with universal constants.
As always these decisions are not arbitrary, at all - just as no art comes from a void creators are permanently inspired by each other and their predecessors. If one's creation is fluent at all to anyone but their own neurons, then it was a product of reasoned decisions that interact with the rest of humanity. They are by definition not a product of unmitigated freedom and arbitrariety. But of cooperation, plagiarism, and inspiration. Creators are not absolute monarchs of their own minds, they are not lights which shines no further than their personal room in the ivory tower. They lead human lives and are in conversation with the rest of the world.