Charisma can keep an ancient dragon from eating a 3rd level party. That's a lot more "do not die points" than an 18 con is going to give a 3rd level PC, or an 18 strength will give a 3rd level fighter, or...
But there are two absolutely critical parts there, one you mentioned and one you didn't.
Can. It
can do that. Or it
can do absolutely nothing...or even make the situation actively worse! That's the first impediment.
The second: You are
going to make attack rolls. You are
going to make perception checks. You are
going to make initiative rolls. That's literally never not going to happen if you play D&D for more than a couple sessions. But you may or may not ever get the chance to make Diplo/Intimidate/Decep checks. If you fight zombies, you don't have anything to negotiate with. If you fight, say, illithids? They aren't going to negotiate with
food any more than you would. You can genuinely have entire campaigns where it just coincidentally never comes up.
Hit points always come up. Initiative and AC always come up. Perception always comes up. Charisma skills, while extremely useful
when they come up (except Intimidate, because far too many GMs make it suck), may or may not come up. Such is true of several other skills, notably knowledge skills and things like Animal Handling where they're very specific in application. I'd actually say Stealth is another "it WILL come up eventually", but it's got a longer time horizon--so for sufficiently short games, it might not come up, but anything that gets past, say, ten sessions? You're gonna roll Stealth at
some point. You genuinely may not ever roll History or Animal Handling--or Deception. To deceive, you have to have someone that can understand you that is worth deceiving. That's not a guaranteed condition. Rolling initiative and losing some of your hit points
is a guaranteed condition, at least in a D&D game.
Charisma is the god stat in that it is the only stat that can consistently accomplish things that affect more than just your immediate surroundings in this fight over here or that one over there. Strength, dex and con aren't going to, outside of very rare corner cases, affect nations, continents or even entire worlds. Intelligence and wisdom have somewhat more potential for that, but still far, FAR less than charisma.
You attribute to it consistency it does not have. You then talk about its
potential, not its actuality--as though that were the same as the consistency. It is not. Yes, I completely agree that Cha skills
potentially can do absolutely enormous, world-shattering things
sometimes, with luck, and GM buy-in, and a good reason, and, and, and.
Con has the objectively universal applicability of giving you more Do Not Die Points. It 100% guaranteed
always does that, in every edition, no matter what. It gives different amounts, to be sure, but it gives them all the same. And since, as stated, you are
always eventually going to make Perception checks and Initiative rolls, you are
going to use those things. It's not a matter of if, it's a matter of when. That cannot be said of Cha.
Some games it can be. Plenty of others, it can't.