I have been running quite a lot of maths recently to try and work out whether different combat models would work in Monitors, let alone what numbers to use. And this has given me a huge amount of sympathy for people faced with doing this sort of thing professionally.
The thing is, it's relatively simple (by which I mean to say, very complicated) to do calculations for what would happen (on average, of course) if hero X and enemy Y stand still in an enemy room with no features whatsoever and shoot or stab at each other. You can, if you want, tweak the numbers to produce different results, or different distributions - maybe you want pretty reliable combat, perhaps you prefer it dramatically swingy. You might introduce a complicating layer of different attacks, or perhaps multiple combatants, but while these make things even more difficult it's still possible to see how you model it with maths, even if the execution is a pain.
The problem really comes about when you try to introduce other stuff. That is, anything that allows combat to be remotely interesting.
In combat, we want heroes to use cunning and stealth and tactics. They take cover, launch surprise attacks, use suppressing fire. People don't fire endlessly at static targets, but duck and crouch and move around for perceived advantage. Large groups try to encircle smaller groups, while the smaller groups try to use terrain and technology to create bottlenecks, allowing them to face only a few enemies at a time.
How much of an advantage can the hero gain by doing this? Exactly how much do we want the hero to do it - or any particular heroic archetype? Should they be taking cover frequently for a small bonus, occasionally for a large one? How successfully should they be able to bottleneck enemies, and how does that relate to different enemy types? How effective is that suppressing fire?
Very quickly, you end up in the situation where you'd have to picture the whole fight in your head in detail in order to work out exactly what kinds of factors you want to come in and how effective you want them to be. You end up with an unmanageable number of things to take into account, and potentially with a massively-spiralling set of rules to handle it all. Or else you resort to alpha-testing each possible set of rules in detail, each time having to decide not only what effects you care about, but the numbers that should be attached to them. Real game designers have my sincere sympathy. I have no idea how they handle this stuff.
Which is to say nothing of Grapple rules.