Probably because the civilizations of the planet Earth are the only ones that interact with other realms. The concept of extraterrestrial life in Earth's plane of existence has never been explored in MK (at least, not majorly). Perhaps it is assumed in the MK universe that Earth is the only planet that has civilized life on it.
If an alien or character from another planet within Earth's realm is introduced, perhaps they would take issue with it being called "Earthrealm", not to mention the fact that they rely on people from Earth specifically to defend their entire universe.i dunno if I'd love a plotline like that, but it would be interesting at least.
