Why on earth would a group of PCs with any sense of self-preservation want to make a combat the least bit more challenging (i.e. dangerous) than they have to?
"Hey, I know, let's walk out of our hiding place and challenge those 20 Orcs to a fair fight! I mean, sure, they could easily kill two of us in the process, but that'd be way more fun than shooting most of them down from here and not taking a scratch, right?" Players who run PCs who think like that really do deserve to have those PCs die. Over and over again, if necessary.
Co-operative storytelling means you're co-operating to tell a story, but says nothing about what that story might consist of.
Yes, but that game is simulating what, in terms of the characters' reality?
Combat as war, where blood flows, survival is job one, and characters (and opponents) die in messy fashion?
Or combat as sport, where dramatic poses are struck, the foes die cleanly, and nobody else comes to any real harm?