Some mechanics are a necessity, if only to keep players from doing things that ruin the setting like deciding on a whim to make up five planets in a RP that takes place across 4 star systems [which turns out to be quite large even when you have spaceships and warp gates]. For instance in a space RP that takes place across 4 star systems a interstellar alien empire suddenly becomes rather plot wrecking if any player could on a whim make one.

Similarly PoW would have turned into a fallout-esque RP should nukes have been a thing and needless to say the subject of gunpowder based weapons in a fantasy setting can end up rather sore even though early gunpowder weapons were pretty shitty. After all, why use a arquebus when a composite bow is just as good if not better?