For a coachbuilder building a full-length, raised roof ambulance or hearse, most of the station-wagon-specific body parts wouldn't be usable anyway. Since they needed to make the roof longer and higher, it was probably just easier, and stronger, to press the roof as a single unit than to try to create something to attach to a wagon roof. As indicated, the side windows and rear fenders would not be useful, also, due to length - and neither would the tailgate, which would be replaced by a rear door. And yes, the station wagon as a base vehicle would have cost more than a sedan, so if you're discarding all those parts anyway, why not pay less. (They would certainly have had a source to resell the unused doors and trunk lids.)
Obviously, the wagons and short wheelbase cars (Seville, Consort) did benefit from station wagon bodies.
