I don't know why the developers made that decision, although the map is certainly cluttered enough without these Historical Locations. It's too bad you could play through the whole of Assassin's Creed: Odyssey ($60 on Humble) without stumbling on this other layer though. There's a lot of interesting historical information contained within, and a lot of great pseudo-historical information as well--insight into the Ancient Greece of Odyssey that explains how and why it differs from the strictly historical Ancient Greece we might know.