The same thing happens with intelligence. D&D characters often have superhuman abilities, so you've got to just do your best with the roleplaying, but let interpretation and die rolls fill in the gaps.
On the contrary, if you keep trying then eventually you'll roll a 20 no matter how low your CHA is. That's good enough in circumstances where you do get to keep trying and you only need to succeed once. Sokka would keep trying while Zuko would refuse to risk looking like a fool.