simulationism, in foregrounding the fiction itself as the thing to be prioritised and enjoyed, can foreground these various elements, but not all of them at once. A game like RM, and the sim-ish aspects of Burning Wheel, foreground the fiction: PC build (and how this models or exhibits the fiction), and action resolution (and how this models or exhibits the interplay between in-game situations and character attributes and actions), and reward (eg BW has elaborate rules for how using skills and abilities, and/or practising skills and abilities, improves those skills and abilities).
Whereas the sim-aspect of sandbox-y D&D is much more about setting, and how it feeds into situation. (As I posted about a long way upthread.)
And here's one reason why you can't prioritise all these elements all at once: if you foreground system, then - of necessity - setting and even situation will to some extent escape your grasp, because you can't control the impact that system will have on it. Conversely, if you foreground setting or situation as that component of the fiction to receive "elevated appreciation" then you are necessarily going to have to subordinate system, as it cannot be allowed to disrupt the setting or situation that is the object of appreciation.
This is another tension that manifest quite commonly in D&D play and discussion about it: see every discussion ever, for instance, about "fudging" or "rulings" or the like to make sure that "the fiction" turns out as it is supposed to.
So anyway, there are some thoughts on simulationism in D&D.