The preamp contributes to that sound, if that is what a person desires (and obviously many do), but Sadowsky pickups are part of that sound, as well as wood selection and other parts of the bass.There are lots of other high quality Fender style passive basses out there. Sadowsky sets itself apart by having a Fender on steroids sound and that’s due to the preamp.
You asked, "Why even buy a Sadowsky if you plan on running it passive?".
A lot of people probably wouldn't, but some people (maybe only a few) are "passive purists", so to speak, yet still love the other aspects of Sadowsky basses.
I would prefer an active/passive switch myself, and the VTC would be a definite requirement.