I've asked myself that exact question multiple times when playing different pen&paper games.
Shadowrun being an example. The 3m tall Troll is less intimidating then the skinny elf because of charisma.
At least in pen&papers there should be rules for: you can intimidate by brute force (lifting a person or an object = strength), imposing physique (constitution) or subtle threats (charisma).
I doubt that this would be a feasible say to do it in a computer game though and that's sadly why we have to use charisma.
