Role-playing games are notorious for rules fluidity: players modify the rules to suit their immediate needs to such an extent that there is often a special rule granting this authority. It may even be a sufficient (though not necessary) feature to categorize something as a role-playing game. I won’t try to sell that point here, but think about it.
So why? Many bemoan this, wanting rules that satisfy all table needs. Many celebrate it, pointing out that this is where the fun is (where the game becomes a game about designing the game you’re playing, which does sound fun).
In general an RPG explores an imaginary space. A space the size of the world or larger sometimes. And the behaviour of this world, no matter how weird, is intuitively understood. We might all understand it slightly differently, but we do have a private understanding. We can easily imagine the way things that are not described in the text would or could work. So the private world of the RPG is effectively as complex as the real world, but it gets explored piecemeal — it doesn’t exist somewhere in whole cloth but rather as you imagine a new part of the space you easily imagine how it must operate.
So now you want rules to govern how this space is explored.
Let’s call the imagined space a BARN.
Let’s call the rules a PAINT BRUSH.
You want to paint the barn. You need to pick a paint brush, The brush, however oddly, must also be fun.
Okay, so you love detail work (a fiddly combat simulator, say, with lots of kinds of polearms meaningfully differentiated). You want to use a double-ought brush. A tiny brush. You can never paint the whole barn with this. That’s fine, you just give permission to the end user to paint whatever bit you left blank however they want. If they enjoy painting, everyone wins. Well everyone who likes detail painting anyway.
But let’s say your goal is to cover the barn. You don’t want to leave any space for the end user to paint. But your imagined space is huge and complex. What are your choices?
You can broaden your brush — abstract your system to the point that you get enough coverage to credibly cover your barn. It might be a little sloppy in places, but you can have a reasonable expectation of coverage. Now your detail painters are still going to get in there with their fine brushes and elaborate and touch up the moulding and so on, but you can’t stop them. You achieved your goal of delivering a painted barn. And depending on how much work you can do you can go anywhere from a 3” brush (pick a system that suits that!) to a paint roller (again I invite you to imagine what game I’m thinking of) to a spray gun. I bet there’s even an experimental explosive painting device out there that paints not only the barn but much of the surrounding countryside.
You can also constrain your barn. This happens a lot in some design spaces: declare we are not going to even think about the barn. We are only interested in the door. Anything outside the door is not part of this project. And then pick a relatively fine brush and paint a perfect door. There are lots of games in this category.
When we say “why are RPGs like this” we aren’t really talking about RPGs. We’re talking about some category of RPG or even some category of player talking about RPGs. And consequently we’re talking about a fairly specific barn/brush case or expectation. But in reality there are many ways to paint that barn, with and without requiring the rules-creative input of the end user. We have to think of handing the end user the brush as an option any RPG should consider — and then accept or reject.
The important thing about game design to me is not doing it one way or another. It’s doing whatever you do deliberately. Everything should be intentional in your text. There may be unanticipated (emergent) properties to the game, but everything you write down should have a goal. Including, possibly, the goal of offering design space to the user.