There are only two possible scenarios for the non-existence of extra-terrestrial life.
1. They simply don't exist anywhere in the universe. We are it as far as we know.
2. They do exist but are so far away as to effectively be non-existent. We can't communicate with them, we can't see them, and the vast distances between us prevent any hope of possibly conducting communications. We are on an isolated island and they are as well and that's the way it's going to be. So they in effect, don't exist.