To the average Star Wars prequels critic, this should come as no surprise. Rick McCallum, the producer of the prequels, confirmed that every single shot in Attack of the Clones and Revenge of the Sith used a heavy amount of computer generated images.
Hayden Christensen also confirmed just how heavy the use of CG was when he said he was pretty sure there was only one actual set he worked on, meaning one that wasn’t just a green screen and props. So it should come as no surprise that there are no physical clone troopers in the entirety of the trilogy.
If you go back and look, every shot with them has that glossed over “CG” look to it. The actor was present on set, however, during scenes that required a clone’s face, such as the Order 66 scene in Revenge of the Sith. Many fans have criticized the heavy use of computer generated imagery in the prequels, feeling the environments look sterile and fake.
Many have criticized the over usage of CG in filmmaking in general as well. Generally though, it’s something an increasing number of filmmakers are starting to do because of the convenience it offers, as shooting on location can be expensive and tiring.