The airborne refers to the thing it is providing early warning of, it refers to its purpose and role, not its mode of travel. This is intended to identify potential airborne threats, like enemy aircraft, missiles, etc. Consider that aircraft can also provide early warning of other things, as widely varied as submarines (sonar) or wildfires, and the "airborne" part should make more sense. Alternatively, we also have airborne early warning radar that are on the ground and are not aircraft at all, and theoretically we could have ships or vehicles that do the same thing. This is the aircraft version.