How many RPGs do you know which consist of a single book? There are definitely some, plenty of indie games especially are singular works. When it comes to the games most people play, though, you can expect that the core rules are joined by supplements, additional books which expand the game through either deepening existing elements or adding new ones. Beyond that, you may have secondary accessories, things like dice, card decks, and maps which add to the physical experience of the game. Taken together these elements create a product line. When you add additional material made by players and designers other than the original authors, then now you have an ecosystem.