*'There are so many subtle variables involved in good acting'

Yep. The main one being unpredictablity. You can give the same lines to 10 actors and they'll all do them differently, and there will be variations according to their mood when they say the lines.*

It's not just that.

If you ever watched "Who's Line is it Anyway?" there's a bit they do frequently where two or three of the comedians improv a scene, usually fairly straight, and then the fourth (usually Colin Mochrie) runs in acting as a director and tells them to do the same scene another way: as drunken ninjas or something.

Now that's done for comedy but it's only a takeoff on what directors really do. "Give me a little more intensity", "Act more betrayed", "Show that you realize what's happening a little sooner in the conversation". It's easy with human actors.

For CG, now, you have to view the scene, go back to the animators with the changes you want, they spend hours or days changing it, view the scene again, go back the animators if you want it tweaked some more...and that isn't going to change until you have an AI that can understand what the director means and make the changes quickly.

Even what's considered the current pinnacle of CG characters, Gollum, depended heavily on Andy Sirkis for physical and voice acting. Gollum could cower because the guy in the suit with the reflective tape cowered in a way that everyone instantly recognized as cowering.

Now, this isn't to say it's impossible. There's some damn fine CG out there with expressive characters who rely purely on the animation. But it's a lot slower than directing a human.