Maxperson
Morkus from Orkus
Because the rules of an RPG do not tell you what that scene is. Nor do they include any way to determine what that scene is. You are expected, as a player in an RPG, to create that shared fiction (whether alone in a trad RPG as the DM, or as a group in other RPG's.) and THAT SHARED FICTION is what you play out.
Here is the rules for Pictionary. What do you have to create to play Pictionary that is not contained in the rules in order to play? You open the box, follow the instructions and play. Done. Nothing, not a single thing, has to be created outside the framework of the rules. Your categories are pre-selected, the words that you have to draw are also pre-selected. For example, your Pictionary will not contain words not in the language selected. You don't need to create new words to play Pictionary. In fact, you CAN'T create new words and play Pictionary. Every single thing you need to play that game is self contained within the rules of that game.
Then substitute Charades for Pictionary. The rules do not dictate what you are to pick for your charades. You can also use Scattergories. The rules say to use the letter C and give catagories, but the players create which words they use. That's the same as D&D rules instructing you to create a scenario, giving you advice on how, and then leaving you to create up with the details.
@Ovinomance has you here, not that he should have had to. It's painfully obvious that scenario creation is just a form of set-up, and almost every other game has set-up.