It would be much, much more expensive. Composites are used for some smaller structures, such as the stack enclosures on the San Antonio class LPD's.
The military cares about cost? :)
As it turns out, they do. Things built to MILSPEC through government procurement are expensive enough as it is without the added expenses you could start to introduce if you wanted. ;)
The point of these ships is to be cheap.
They make some fairly large yachts out of fiberglass.