If one believes that the universe exists as an actual physical entity, with matter and energy, then aliens must exist. Such a massive universe cannot exist with only us as sentient beings.
However, if one believes that the universe is a perception of human souls, then aliens are not a given. If the Consciousness within our sphere of existence decides that we need to experience aliens, it may bring them in as perceptions created by our human souls, just as it may bring in angels. Or it may bring in just perceptions of their dead bodies from supposed past times and broken spacecraft, in order to whet our imaginations.
OR, perhaps there aren’t just human souls in this sphere perceiving the universe, but also alien souls that perceive their own existence as well as the universe. Or, perhaps we are perceptions of alien souls, just as angels are perceptions of human souls. You can see how convoluted this can become. I can consider any of these hypotheses, but until live aliens become a truly known phenomenon, I’m sticking with the concept of human souls, and human souls alone, perceiving this universe.