Rules of some sort actually have to exist in order for us to fairly play a game. If there are no rules, if it is all continuous ad-hoc invention that can be changed under my feet as a player at any moment, then we are not playing a game. You're telling me a story, but manipulating me into thinking I have some say in it.
RAW is a vital part of RPGs, just as much as tinkering. If there is genuinely actually zero RAW, any difference between D&D and Let's Pretend vanishes.
Also, creating your own settings...isn't a RAW thing at all, like...settings are entirely outside the question of RAW and rulings and the like.