I think that the terrain is designed to eventually generate back to trash. If you kept lowering the terrain, then there would make unnecessary holes in the game. Also, I'm not sure if you're inferring that resources should deplete, but that sort of feature would cause many problems. I believe the current system scatters random materials in junk piles, but they can be temporary depleted if players keep picking for them in the same area. This is probably just a way keep players moving around rather than having them suffer from boredom. The reason it probably turns into dirt, is to indicate that there isn't any materials for you to pick from which makes sense because if nothing is there, then why is there still trash on the ground? I think this is a case of gameplay vs realism, gameplay needs to win this battle. Material depletion will be critical for tribes, towns, and just about everyone. Holes will also ruin the environmental appeal.