my guess is the precision, dexterity and energy cost of finger / hand / wrist movements make them more effective than using a mouth.

An example would be complex poses such as the "gun" shape with a hand as opposed to "pouting" your lips, there's so much opportunity to finely tune the motion from open hand to gun shape but it's more linear with pouting. However I'm not sure where or how the "thinking" part would play into this, eg physical muscle movements vs intention of muscle movements.

And there might just be too much noise and involuntary mouth movements that would make it impractical (eg, breathing, swallowing), plus the fact you'd look like a psychopath mumbling to yourself.