Well, they didn't think they needed to that at all. They figured people would want to make their own worlds (same idea D&D had, too). So they made the spinward marches.Yup. Totally agree. To fill in the vast quantities of blanks we got random dice rolls. Now we are left with the flotsam of randomness and gyrating logic to even try to justify the existence for many locations.
I think, in hindsight, they should have left it alone until they were able to better conceptually plot out major explored areas.
Sometimes smaller is better.
Much of the rest of the material was made by other people, whether fanzines or third party publishers.