Heh. Well, I'd even point to D&D here. You read the PHB, cover to cover, and you can make a character. True. But, you cannot really use that character without a campaign. So, you go to the DMG. Now there's all sorts of idea seeds in the DMG to get you started on creating, but, even then, since nobody even reads the DMG ((

)) it's not really necessary.
I mean, could you look at the 5e rules, including Spelljammer, and predict that my Spelljammer campaign takes place in a knot in the Astral Sea known as the Gyre, where travel between points is difficult and restricted to the streams between points? Or that the Gyre was created by a war between invading Formians and the various races inhabiting the Gyre who turned to Clockwork warriors to defend their homes, culminating in a massive magical conflagration that tied the region in knots, unmooring it in time and space so that traveling to the same location and time is nearly impossible?
None of that exists anywhere, as far as I know, in the Spelljammer books. Yet, I don't think anyone would claim I'm not playing an RPG or playing D&D.