No games come with manuals anymore, or at least a printed manual worth a damn. This is the era of digital distribution. Even if you still get it on a disc in a box, there's nothing in the box, but cover art.
But look, we get it, this is the post-print era, print's expensive, just put that stuff in a damned PDF that installs on your hard drive along with the game files.
... OK, I admit that might be PC master-racism again, as I don't see a gaming console solution, but still. 
... I don't mind if the Prima game guide is where those mechanics are found, I usually buy it, and avoid like Sylvius reading most of it except for mechanics until my 2nd play through. Then, because I'm a whacky completionist and want to see everything I missed on my 1st play through, I read the guide to find out what I never saw or did, and try to see and do it.
But yeah, it's not really right to make that the only place to find good mechanical documentation, as opposed to walkthroughs, quest guides, romance guides, where to find items, etc., etc.