Scenarios are generally specific to a particular set of rules. Unlike simple adventure novels, modern commercial scenarios require access to, and understanding of, complex rules. The rules make it possible for a story line to have literally hundreds or thousands of choices rather than the three to twenty actual choices available in the books. Rules also allow referees to design scenarios. Importantly, the rules provide a structure that takes all arbitrary choices away from the referee and gives those choices to a table or a statistical range.
Rules make control possible.
Scenarios make play attractive. Dungeons and Dragons made the play very accessible. For comparison, a game where characters go around and make friends would be possible, but would be hard to structure so that it contained the four elements. Successful variants are games where one builds chains of hotels, competes in the computer business, explores the universe to prevent a star from exploding, etc. Such games can be simple.