To me, the platonic ideal of a tabletop role playing game is this- a kit that is perfectly suited to building stories with a particular mood and theme.
Ideally, this hypothetical perfect ttrpg won't even require you to know what the theme is! Instead, it will provide you tools (mechanics) and materials (setting) that, in whatever way you put them together, will lead you to creating stories with the system's intended themes!
Of course, platonic ideals are, by their very definition, perfect, and thus impossible. So it makes sense that most games that aim for this ideal will need to signpost what sort of mood they want you to create, or even outright state it directly; an instruction booklet of sorts, to go with the kit metaphor.
But, it's definitely possible to attempt the impossible!