*AIR*craft are craft that need & use air. Rockets not only do not need air, they prefer a lack of air. So they are not aircraft. They are spacecraft (they are designed for and prefer space)

To clarify they use air to generate lift and hence the ability to travel in the Z-axis, not that they need air to make the propulsion system work.

Edit: if you strapped a Raptor engine to a 747 it would still be an aircraft.