The Taijitu map as currently constructed is understood as being part of a world. Yes, you could reach the other side of the map by sailing off the border and around the world, but there are several problems with this. First, the world is a big place, and if you try to circumnavigate it any conflict would probably be over before you could reach it in this manner. Second, the other areas of the world belong to other regions, and they will use all the IGNORE power they've got to prevent you from sailing/flying/marching through their territory.