Get me a sample of one alien, then we’ll talk.
DemForBush mentioned the possibility that aliens would have likely evolved from conditions that would be too hostile for the likes of us. If so, then Earth would almost certainly be too hostile for them.
Wouln't that be funny - if there were intelligent alien species who gave this planet a wide berth because they couldn't use anything on it. That would knock a lot of science fiction right into the "science fantasy" category.