My guess is that the movie studios are more concerned with being a successful business than making good art. As a business that wants to make the most money possible they cast big name actors to help get more people in the seats, regardless of the historical accuracy or ethnicity. Another issue may be that big name actors might have more power when it comes to which movies they do. "If Bale wants to be Moses, let him be Moses!"

And that's the beauty of fan casting! We can let the voice of the people be heard, so any discerning casting directors can reference myCast to for better suggestions.