Thats why the PT gets so much criticism for being "fake" and "artificial" and "CGI-happy". A lot of the actual objects and environments are practical elements and models but it doesn't make a difference. The same with all the plates. Most of the BG's and textures are based off real photographed plates but it doesnt seem to have made a difference since there is still an artificiality to it. The reason why all these methods are used instead of actually filming them is purely cost savings. Its a $250 million movie made for $115 million. The battle of Kashyyk looks like a freaking video game compared to Saving Private Ryan and there was nothing stopping Lucas from achieving the same realism as that film except the fact that it would cost another $5 million.