"It is a dissociative symptom of many conditions, such as psychiatric and neurological disorders, and not a standalone disorder."

"Derealization is a subjective experience of unreality of the outside world, while depersonalization is unreality in one's sense of self. Although most authors currently regard derealization (surroundings) and depersonalization (self) as independent constructs, many do not want to separate derealization from depersonalization."

Thanks TheByzantine. So I guess since derealization can be a symptom of so many things, doctors don't want to make it a separate disorder? Makes sense.