Because there is no viable evidence that says other wise. We as humans have only been able to explore about 4% of the universe which means theres still a whole lot that we dont know. If we are able to breath and live on this world whats to say that their arent any living breathing creatures on any other world that we have yet to identife. A good scientist doent cross out any possible outcomes without all of the facts, and the fact is that we dont know enough knowledge to make those accusations.
