There are a number of quests that explain the weather and plant ecology in E. Than. near he goobue corpse. The short answer is it used to be bone dry for decades, hence the name, before the calamity which was only 5 years ago story wise, and they just didnt rename it. In those 5 years its progressively gotten more rainy. Not long enough to water rot all the cacti, but short enough the forest is only starting to clain the desert through loamfall enriching the sandy soil into something usable.