This does not lift my confusion. You have been presented a repeat, observable pattern in cinematic combats: not every outnumbering force member attacks every round but a random number between 1 and all of them does.
I think this is part of the problem of our miscommunication. It is not a random number. It is exactly the number that the director/writer wants. That is not a pattern that's easy to emulate in a game. And the answer to why we wouldn't becomes obvious at that point: it's not a game any more.
Sure. In the case of "One v Many" with round-robin attacks, generally every character in threat range will attack each round. Especially player characters. Imagine a PC not attacking deliberately and then another PC dying. Hard feelings are bound to ensue. That's why you need a mechanism to keep a random number of "outnumberers" from attacking. Not just a random number but probably a random and each round changing subset of them.
I understand that it's completely unnatural not to attack when you could, and when it makes sense to do so. What I'm not understanding is what this mechanism would look like. That's the kind of example I'd like to see.
1. GM: You are attacked by a giant killer robot.
2. Fred: I attack the giant killer robot with my shield and magic helmet.
3. Barney: I watch Fred attack the giant killer robot. I'd rather wait until Fred is defeated, that way I can be defeated myself. I've seen this happen in movies, and it's cool.
4.
5. Profit!
(3) is mostly a joke, although I've certainly seen GMs think players ought to act that way. The question is, why would they? They can act that way now if they want, but of course they don't want to, thus this thread; and as you said, there are going to be hard feelings if they do. So I want to see your example that rewrites (3) in a way that is actually fun or at least interesting, and puts in (4). Then we can get to Profit!
I'm assuming for the moment, but you can correct that assumption if I'm wrong, that your mechanic isn't simply roll a random number from one to the number of player characters; that would not just force some players to not attack, but would also force other players to attack. Somehow this mechanism needs to make the game fun not just for the players to sit it out, or split to have their characters do something else, but also for the players who do not.
1. The pattern is not faithfully enough replicated.
2. We shouldn't replicate the pattern to begin with because will do bad thing X to our games.
Right, and that's the problem. As I see it, the more faithfully you replicate Hollywood cinematics, the less agency the players have over their characters. That's the bad thing that will happen. In Hollywood cinematics, we're mostly expected to ignore that the characters are just puppets held on chains by a single individual. The cinematics you think are cool seem to me to be exactly those places where this is hardest to ignore. And in a game, it seems to me impossible to ignore. So that's why I need to see you give some example of a mechanic that you think will actually do what you want. It's also why when actual examples are not forthcoming, you get the reaction you say you often get. Because the only way to get what you want is to take control from the players and give it to the GM. So far, the terms you've used to describe what you want are emulating actual scenes, fashioning a particular outcome, and choreographing the moves of the player characters.
To relieve your confusion: I am in fact saying that in order to get the Hollywood cinematics you want, it looks to me as though you need to have a script. I'm asking for some sort of real example to show me that I am wrong. It could be mechanics, it could be a fake play example like above, or something else. But so far, the more you've described what you want, the more it seems that the only way to get it is to have a script, possibly generated randomly just-in-time, that the players must have their characters follow.