Except, you really don't see Mickey. You see someone in a giant Mickey suit. We've been trained to think that that is a fairly good representation of the cartoon Mickey, but if you really did an objective comparison, you'd see how off the costumed person is because you don't have 3 foot tall people with impossibly thin necks and limbs available to portray Mickey. And you don't see realistic portrayals of dragons or maleficent because of the impossibility to do that realistically... for now.
And so, you have the same problem with the Na'vi. You don't have nine foot tall people with impossibly thin necks, waists, and limbs (thanks to lesser gravity) to portray them. And a suit would make it worse.
Sometimes the IP makes it impossible to portray certain characters in the real world in such a fashion that they're walking around amongst us.