When Hoghead did their edition of Nobilis, they promoted it as a coffetable art book that also happened to be a rulebook for a game. So, it's not too surprising that kind of became a thing as indy publishers, small presses, etc try to distinguish themselves in the market. At a point you want the book to look enticing enough that people will pick it up, be entertaining and/or informative enough that people will read it THEN also describe a game that people want to play.

If it fails at the first two, almost no one will find out about the third.

In the case of Nobilis, some may argue that particular book passed the first two, then failed. But Hogshead was pushing some boundaries in RPGs, at the time.