In spite of Abram's desire to do more "practical effects" it still comes down to money. Flying spaceships are much cheaper to do CG than practical. The cost to build a CG model vs. a real model is about the same but the cost to shoot real models is a lot more expensive than that to render the CG model. You can also render the CG elements for a multitude of shots at the same time whereas with a real model it's one at a time making for a longer production schedule.
There's a lot more to the decision of whether to use practical miniatures or cgi than just the cost.