Do you know that gods don't exist? Based on what? I bet the same reasons I don't believe gods and aliens don't exist.
On the contrary, we know what sorts of things would constitute life on other planets and the type of evidence that would show the existence of such. We are still learning the conditions required for life to develop, but those that we do understand seem to be common in the universe. We also know our technology is not to the point where we would expect to find evidence, even though we know what sort of evidence would be required.
In contrast, few seem to be able to say what would constitute evidence for a deity or precisely what is being looked for. Those who have conducted investigations into the 'supernatural' have found no evidence of such, even when such evidence was expected. Furthermore, it is not simply a matter of technology, but rather even defining what it means to be a deity at all.
So, at this point, it is premature to conclude that no life exists on other planets, while it is reasonable to think say that the term 'deity' is too poorly defined to be even a subject of investigation.