What about all the other lifeless planets and moons we can observe in our own neighborhood?
They fall out side the life zone and/or are not the correct size to sustain an atmosphere. We are just beginning to explore our own neighborhood. I remember a professor telling us once that he never expected us to find any extra solar planets. Well, we have more than 100 now. For the record, I do not believe there are life bearing planets out there; however, I do suspect there are.