Pathfinder in particular is booming because all of its game content is literally open source... inherited from the D&D 3.5 "Open Game License".
There are plenty of online databases and apps that get you the entire rulesets of Pathfinder. Paizo's official one is here: http://paizo.com/pathfinderRPG/prd/
You can literally play all of Pathfinder (including all of their optional rules) without spending a single dollar. But the books, adventure paths, and art is so lovely that the GMs and hardcore players are willing to buy them up anyway.
There are plenty of online databases and apps that get you the entire rulesets of Pathfinder. Paizo's official one is here: http://paizo.com/pathfinderRPG/prd/
You can literally play all of Pathfinder (including all of their optional rules) without spending a single dollar. But the books, adventure paths, and art is so lovely that the GMs and hardcore players are willing to buy them up anyway.