Given that the essence of a RPG is establishing a shared fiction, what else would it be?
There is no "what else". They're separate terms.
And yes, it is shared fiction, but what you're doing is inappropriately conflating the GM's
share of authorship with being a violation of the Player's agency when they don't ever actually interact like that.
You're essentially conflating the agency of the Players as actual human beings living in the real world with the agency they are exerting through their characters in the context of a gameworld. Thats the difference between agency and authorship.
In a game with a more equal divide in authorship between the GM and their Players, Players will be able to exert agency beyond that of their character's agency, and thus
author parts of the world into existence, and in the context of narratives, thus directly co-author that narrative.
But, as said, that has nothing to do with agency.
That does not, however, mean that a trad game is all GM authored. The GM actually has less authorship than you'd think, and thats expressed in the myriad ways trad games provide players to interact with the gameworld, regardless of whether or not theres a story involved.
These systemic interactions are what lead to emergence, not just of gameplay, but of narratives. This emergence is what stops either player or GM from wielding all control in a given narrative, because now they have to share authorship with the system itself.
Thats the real value of more robust games that can provide a lot of mechanical interactivity, as they are fundamentally unpredictable to some degree, and when done well, this unpredictability is what results in a great deal of captivating fun.
In games without this robust support, this unpredictability is hard to achieve, and many who reject it outright also do lose the ability to truly be unpredictable.
Thats the setback of trying to wrestle away authorship, as authoring some element doesn't have the same effect.
A solution to a given problem that arises organically as the sum of a near infinite amount of choices and systemic interactions is virtually always more compelling than one thats written bespoke for the situation.
Thats why all the best and most memorable parts of DND all come out of things that nobody at the table could have foreseen. But when you're just writing bespoke solutions outright, its always predictable. You always know the problem will be solved, and may be you still get a kick out of someone being creative with it, but it'll never hit in the same way.
Put another way, this can actually be likened to cheating in video games. When you remove all constraints, fun
can still be had, but it can and does also ruin the fun of playing normally, sometimes even permanently.
The loss of systemic unpredictability effectively does the same thing.
And this, for the record, is actually a big reason why I do not like story first/now games. Rather than resolving the issues of bad gameplay, they just delete it in preference of generating ultimately shallow facsimiles of the desired results.