Well, the out-of-game reason is probably that the dev team wants to save a full reveal for later expansions.

In-game, it may just not have been extensively explored in modern times. Maps probably exist, but not publicly (betcha Limsa has a bunch though). Also, remember that Garlemald owns half of Hydaelyn; extensively charting Ilsabard and Othard is probably difficult when we're at war with those who own it.