rules

I think rules in combat sports fall into two categories:
I. Rules for safety
II. Rules to make the other guy admit that you won.

Category II only exists competitively, and most often as a consequence of safety rules, in order to prevent game-breaking tactics. But what that's really necessary is a sense of "fair play"- meaning, in this case, that both people share the goal, and avoid doing the things that are only possible because you're not trying to kill each other while working towards this goal. For example, if you slow down, there are tactics that are possible that would not be at full speed. Someone speeding up is obvious, but not obvious is the case where an action would only be possible if predicted. I start to swing, you counter-attack, I change the course of my swing to block your counter-attack. At speed, this is generally only possible if I planned it ahead of time- if you hadn't counter-attacked, i would've done it anyway. but with slower speeds, you can do it seamlessly, exactly as if you had planned it. And the only person who could know is me. You can't use rules to get rid of this problem- you either would have to go full speed, or else both people have to assume each is playing fair.

and then, competitively, you get the unintended consequences. In foil and saber fencing, they have rules of right-of-way to settle the issue. If I attack, you counter-attack, and we both hit at the same time, I win. Originally designed to promote good sword self-defensing technique- you saw the attack, and you decided to attack? Dumb. You get punished. Which, good, right? Except then competitively, the attacker gets REWARDED when his opponent counter-attacks. And the more committed the counter-attack, the better it is. So you get attacks that start long, lazy, and with the head and torso wide-open (but not the arm, because the opponent could hit that and get away) that encourage the opponent to counter-attack.
Safety foundation ("Only counter-attack if you can avoid getting hit!") plus who-wins rule ("Attacker wins vs. counter-attacker") = reality warping behavior ("welcome, suicidal counter-attackers!")

With the foam combat I've been doing, the head is not a valid target, and if you intentionally (or even solidly unintentionally) strike someone in the head, you should take a loss. Which has some predicatible warping effects on technique, as there's no reason to learn to especially learn to protect the head. But then the warping effect gets stronger- one of the essential problem of a large shield is that you can see, or you can protect your face, but it's hard to do both. Problem removed! Just stick your head right over the top of that shield. Which, naturally, people do. And but then it reaches even further. To counteract the advantage of a big shield, you can "break" it by hitting twice with a two-handed weapon. Which works sort of okay in battles with multiple people. but (unintended consequence), one-on-one, sword'n'shield vs two-handed weapon, the game is trivial (meaning, only one real chain of events, uninteresting). Any pauses mean the 2h-er will hit the shield. If they can hit it twice, the shield-user will probably lose. If the shield-user charges to close distance, the 2Her will have time for only one shot. The shield-user can easily block all shots to the arms and torso during that time, leaving the only possible target the legs. So, the only real course of action is: shield-user immediately charges, 2h-er attempts to hit a leg. If they hit a leg, they win. If they don't, the shielder user closes in and wins.
TLDR version: Two-handed weapon vs. sword and shield is a boring fight, with no possibilities or variations. Which is sort of mind-boggling, given how many possibilities there should be with that matchup. And then origin of that was a simple safety rule, "Don't hit people in the head." with associated who-wins rule, "If you hit someone in the head, you lose."

Every combat sport has these things. Not exploiting them is generally an insurmountable competitive disadvantage. It doesn't take long at all for techniques to warp to adapt to these holes. Attempts to correct the warp will just create new, weirder warps.