Tim Urban gives 10 scenarios why aliens can be out there but we haven't discovered them yet. Give his piece a read for further explanation.
Super-intelligent life could very well have already visited Earth, but before we were here.
The galaxy has been colonized, but we just live in some desolate rural area of the galaxy.
The entire concept of physical colonization is a hilariously backward concept to a more advanced species.
There are scary predator civilizations out there, and most intelligent life knows better than to broadcast any outgoing signals and advertise their location.
There’s only one instance of higher-intelligent life—a “superpredator” civilization (like humans are here on Earth)—that is far more advanced than everyone else and keeps it that way by exterminating any intelligent civilization once they get past a certain level.
There’s plenty of activity and noise out there, but our technology is too primitive and we’re listening for the wrong things.
We are receiving contact from other intelligent life, but the government is hiding it.
Higher civilizations are aware of us and observing us (a.k.a. the “Zoo Hypothesis”).
Higher civilizations are here, all around us. But we’re too primitive to perceive them.