Lemme guess: it all started out as a simple, sane idea, which was then shoehorned into every possible library and function?
They used to have just the reducers. A pyramid of pure functions crystalizing immutable structures into the single application state. It was a real functional programming system. A pure system. The perfect system.
Then someone realised: "Hey, what about when we want to get some actual data into our perfect system. You know, like through... ajax requests?"
Thus, sagas were born.