But they're still not very good compared to the Wizard, Cleric, or Bard who start with 16s (or better) in Int, Wis, and Cha respectively.
They're not supposed to be as mechanically good as the Bard, etc. at Persuasion.
Now if the DM is going to rule that mechanics mean everything, well that should be established at session zero. You would make it clear that the 1st level wandering Bard with 16 CHA basically has more sway than the Emperor with 12 CHA. Role play, background, character experience or fame? Don't matter. Some suave looking stranger at a bar can persuade some devout wife to bed with him instead of her husband because he has Persuasion and a higher CHA than the husband? Who cares. Everything comes down to the d20 roll.
What about the high level wizard who dumped Strength but now claims he's into fitness and the gym? Even spends and ASI and feats to take Athletics and gets to 12 Strength? If he whines and says the tavern bouncer is still more swole than him? Well tough luck.