Rule 17 Of Fiction

Rule 17: Anything can happen in the realm of fiction.

 Rule 18: Don't ask about what happened to the other rules before 17.

Rule 17 Of Fiction is a statement that anybody who works with fiction should live by: anything can happen in the realm of fiction. Rule 17 Of Fiction can be compared to Rule 34 but with less porn: "If it can happen, it eventually will happen."

Why Rule 17 isn't always used
The story. If not for the story, you'd have people throwing fireballs, flying, destroying planets just by thinking about it, summoning dinosaurs, being invincible, breathing underwater, instantly killing their enemies, firing water out of their hands or mouths to cancel out the aforementioned thrown fireballs, bringing in characters from other franchises, and ruining the fabric of reality. In addition, not all franchises in fiction want to be outlandish and give everyone unlimited powers. If everybody had God-like abilities, there would be no story, no one would struggle, there would almost be no need for conflict, and nothing would make sense. So even though Rule 17 applies to all forms of fiction, if it is used to cause chaos in a franchise, it would destroy any form of logic and, in turn, reduce that franchise to a mess.