10 Theories Why We Haven't Discovered Aliens Yet
7. We Are Rare
According to this idea, humans are both the first creatures to survive and the only ones. Allegedly Earth is unique among planets, though scientists are not sure how the Earth could have a quality that makes it better at supporting life than its peers. It doesn't necessarily state that alien life does not exist, but the odds of it are greatly reduced.
This has its own problems, as astronomers have discovered a good many planets with a lot of the features that makes earth habitable, and are confident that the ingredients used in the life concoction found on Earth aren't rare at all.
On the other hand, science is far from knowing everything about Earth, so it is possible that while they think planets like Earth are a dime a dozen, it has an unknown characteristic that puts it above all the rest.