Why do animated CG characters always feel fake?
Because animators haven't fully bridged the uncanny valley. 

There are a ton of little details you notice subconsciously that hasn't been fully mapped out.

Things like how muscles and tendons move under the surface, how clothing and hair behaves as you move around. Even how you adjust your stance as you move your arms around, prepare to move, etc.

Even with mocap it is difficult to capture and recreate all that. It is also not trivial to capture the movement of the fingers.