"Takes two to tango". A very old idiomatic expression that simply means that "stuff" doesn't really start happening until more than one person is participating. We can daydream all day long in our basements, living rooms, garages or what-have-you, but until we get off our butts and do something about our dreams, they amount to nothing.
RPGs are not very different. I spend a lot of my free time world building in my gaming closet, I draw maps, I study modules and publications to get ideas about monster and NPC design, I read novels from various fantasy authors for inspiration, and I play old-school CRPGs and fantasy board games to get a glimpse of where RPG history went. The result of these things are a collection of great gaming worlds.
But they don't matter. They are just props. A collection of props that I put a lot of TLC into, props that have lots and lots of potential, but they're still lifeless set pieces stored away in the closet of imagination (or in my binder) ready to come to life if someone decides to use them. Me or someone else. These things are not what makes an RPG. They could be used for anything at this point: I could write a fantasy novel with them, code a video game, maybe try my hand at a amateur fantasy film and put it up on YouTube. They don't have to be part of an RPG.
The key ingredient to an RPG is the players themselves. When they make decisions in the game, these objects come to life and are given a story. In my games, my players get to make decisions to affect, direct, destroy, or enhance those objects. When they walk past a street in my imaginary village, suddenly all of the dead NPCs come to life, suddenly the town drunk is yelling obscenities while tripping over himself, suddenly the guards are on patrol, suddenly the world has come to life.
Sometimes as a DM I act as a player, but I have the whole world at my disposal, not just the props where the characters are wandering through. The assassins' guild is carrying out missions, the caravans are getting attacked by giants in the mountain passes, the king is getting played by his power-hungry advisors. Many of these things never reach the ears of my players until much later, or never at all. But I breathed life into them nonetheless, they existed and lived and made decisions because I, as a DM-turned player made it happen.
Are my players forced to know about these events? Of course not. Because if they were forced to, they would no longer be role-playing. They would be READING MY SCRIPT. They would be under the illusion of role-playing, but they would be simply following a novel written by me. Any game that forces players down a road that they have not made a decision to follow is not a Role-Playing Game. They've lost their role, whether they know it or not. They are merely witnessing a group of characters, like Frodo and Sam, taking their tour through someone's canned world, adding in their input to humor themselves that really makes no difference to the end result.
Am I dissing on my game world? No of course not. I once posted about the Lovecraftian approach and how we should view our worlds as perhaps actually existing. I don't take that back. Our worlds should be role-played as if they are real. We should take our role-playing with complete seriousness not taking in meta elements from our own world. When we have entered that world, it should feel like it's something real, but when we leave, it doesn't continue living unless we're there to make choices in it. It goes to sleep, it freezes in time, the lights go off....that is, until we return.
RPGs are sandboxes, they are created from player decisions creating life out of NPCs, weapons and places that are nothing more than words on paper until they are acted upon.