Because its full of EXPLOSIVES.
In the modern world humanity is everpresent. Nature only rules fully in the margins left by our mistakes. The minefields go untouched. They can be cleared, and the mines will decay. But you can never be sure. Flooding and rainstorms move the mines around. There is no set period of decay. We can never walk there again with perfect certainty.
The DMZs, Minefields, Chemical Fields, Underground-Coal-Fires and Chernobyl Zones are the new wilderness, shaping the margins of the world with our mistakes.
So we ask the D&D Autumnal world. Why are there so many empty spaces? If empires fall and rise with such felicity, why only points of light?
Too many margins and too many mistakes. Over long epochs of human history the marginal zones invisibly expand and interlock. We are squeezed ever closer. The wilderness is empty because it is dangerous. We made it that way. We cannot remember why, and we cannot go back.