That's not correct; they do in fact carry DDG hull numbers.
The most accurate answer to the OP's question is probably that we're not really sure what we're doing with this class. The Zumwalts are new and wacky and different and challenging and exciting in a way that no ships have been since the immediate postwar era, when technology and mission requirements were changing at a similarly rapid pace. As such, there were a number of two- and three-off classes at the time (Norfolk, Mitscher, Claud Jones, etc.). These ships served with varying distinction in the postwar USN, but in most cases there was never really any intention of replacing the massive numbers of Gearings and Sumners with these designs. They were service-ready investigations of new technology and new concepts. This is probably an appropriate forecast of DDG-1000's future.