Whereas something that is the product of one's own imagination is not an object of discovery.
Now even good storytelling games (eg like A Penny For My Thoughts) have techniques to mediate between just making up one's own stuff, and engaging with or integrating the stuff that others imagine. And once we get to RPGs like (say) DW or BW, there are very sophisticated design features that exploit the asymmetric participant roles to achieve this sort of mediation and integration. Most posts from
@Oofta and @Lanefant don't acknowledge those design features, and the associated techniques, and so just presume that if the player gets to exercise imagination then the player is just making it all up - but that's simply a result, as best I can tell, of not having any or any real familiarity with the games in question.