Hi Sascha,
I am guessing; no reason. It seems to be inconsequential.
Maybe the idea was to leave the Origin behind, so it doesn’t get in the way visually, when Sculpting, but in Sculpt Mode, the Origin (and 3D Cursor and Outline) do(es)n’t show anyway.
In an earlier version of the Fundamentals of Sculpting in Blender, Kent actually uses the default Cube and moves it up in Object Mode.