So other than the fact Oda is a hack and ain't give a f anymore about the curvature of black haki scribbles, I have a theory
Using up or infusing Conqueror's haki drains your life force. That is why Luffy and co don't frequently spam it unless it's absolutely necessary or it's an extreme emotional situation
We know that draining haki is legit. Gear 4 forces Luffy to use up all of his haki in one go and he needs to recover
Using CoO at the highest level weakens the longer you use it in battle without rest
Enma enjoys dat haki suck. If you don't regulate its haki consumption then it will drain you dry until you die
CoC is the embodiment of the spirit of the user, inherent to only a few. So naturally CoC should have even greater risks associated with its own usage like chipping away at your life if you keep using up that super rare spirit
Does that mean CoC users can fully control it at any given time? No, given CoC is unlocked randomly through an extreme emotion or life-and-death situation. When two CoC users meet it's like they are being sized up, they are in that "zone" so to speak and their CoC leaks out
Roger and WB ended up being sick to death. Kaido is a suicidal son of a bitch. Shanks is still young so he can use it freely but he is a Celestial dragon so he might also kinda detest himself enough he don't give a f. However Kidd was about to wipe out his fleet so obv he had no choice
BM looks good for her age but she also isn't a haki spammer, using clear ACoC only once throughout Wano. Same with Garp who is more CoA focused
And Luffy used it so much in Wano he felt his life span was cut a non negligible amount so now he's trynna regulate its use. We already know his life span has been cut once by Ivankov
This also explains why Rayleigh didn't teach Luffy about CoC infusion, he knew there were massive risks associated with it. Probably Zoro realized this instinctively too
Idk why we are trying to justify Oda's garbage power system but it is what it is