A wise designer once commented: "Not many people are able to get more than one golf hole in their head at a time..." I think that is the crux of the routing "genius" — as designers we are expected to keep a lot of balls in the air all at once, and this is not easy.
It is made even more complex when you are walking at the same scale as the land, across the terrain, unable to transport yourself high up into the sky and look down on the possibilities left, right and below. This is why the topo map becomes important — on sites where you cannot see X from Y you often need a map to show you where you are, and to keep things in perspective.