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?
The heck you on about?
It is a decision at the "what rules are we using to play the game" level, not the in character during play level. No one is talking about the PCs deciding anything. The PCs don't make decisions about what the rules of the game are, or how the GM makes encounters, or what abilities the enemy has, etc.
"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.
This comment is beneath you. I don't believe you are the type of grognard that belittles people just because you haven't' grasped the finer points of how they play yet.
Co-operative storytelling means you're co-operating to tell a story, but says nothing about what that story might consist of.
Okay? Who said it did?
The point is literally that the game as it is run these days is about cooperative storytelling first, simulation a thousand years behind that, and player skill in varying positions along the spectrum between the two.
And because it is about cooperative storytelling and not that old school vibe, people expect the game to present
interesting encounters rather than cakewalks. People find original 5e ambushes boring because it might as well just be narrated as a success without them doing anything. If I am roleplaying hunting in the "for food" sense, don't make me spend three rounds shooting the mutant dear, just take my success at ambushing it as a success at hunting it and move on. I am not here to have my time wasted.
And literally when I run the game, if the PCs get the drop on someone so completely that there is no chance of any outcome other than a cakewalk, I don't call for initiative, I just ask them how they take out the poor sods and then carry on with the scene.
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?
Neither. It's a game. It isn't simulating anything, we are playing a game.
If you insist on any roleplaying game action to be some sort of simulation of something, then still neither. It is cinematic combat. The stakes are "real" within the fiction and in the sense that PCs can die, death isn't messy unless everyone at the table actually likes their action scenes gory, no one is striking dramatic poses (what an exceptionally weird thing to suggest), and there are often stakes much bigger and more important than individual survival because the PCs are quite often (possibly most of the time) heroes.
Lets take Terminator 2: Judgement Day as an example. When people die, the camera doesn't linger on them, it doesn't show innocent people dying when things blow up, it doesn't show limbs flying, people screaming for hours in prolongue pain as they slowly bleed to death, etc.
But when Dyson dies, the moment is real, the stakes are permanent and matter, and absolutely nothing about it is "sport".
That is what we are talking about, not whatever strangeness you have concocted to argue against instead of what I or anyone else actually said.