I can think of a couple of reasons. First, if they showed up on the main map, the Arbor Call and Lay of the Land abilities would have absolutely no purpose. Secondly, the map would become VERY cluttered with mining/botany icons, as there are many areas with three or more groups of nodes. Lastly, part of the appeal of gathering jobs is getting out into the world, exploring, and discovering new places to gather; that sense of exploration would be diminished if you could just open up a map and see them.