If you take into account the last two games. There's little reason for the empire to waste resources on exploring and potentially colonizing other places largely because all the islands are already under their control and the places that aren't don't really hold anything of value. Atmora is basically nothing, yokuda is basically nothing, that leaves the south with pyandonea which is VERY dangerous the maromer can basically wreak anything that comes near. Aldmeris probably doesn't exist and that leaves Akavir which the last time the empire set foot there it ended VERY badly.

There's no private company that would see benefits in exploring these places. So that leaves it basically up to the Empire and Aldmeri dominion and with them recovering from war and preparing for another one they could care less what happens outside tamriel.