Possible reasons: 1. Wormhole technology or faster than light travel aren't possible, so we're spread too far apart to meet. 2. We're among the first to have become sentient. This ties in with 3. 3. Conditions on earth are affected by a large number of factors. The drake equation predicts possible life supporting planets and this could be said to give no indication of how many might evolve what we consider intelligence. It could be that life is abundant but our level of intelligence is exceedingly rare. 4. Any race with sufficient technology to visit us would easily be able to hide from us. We already have stealth technology, imagine what we'll have within just a hundred years. They will be orders of magnitude better equipped than us. 5. We might not be of interest to beings advanced enough to visit us. They could have seen many species already and lost interest. This ties in to my next thought. 6. With a high enough level of technology, aliens might completely lose interest in the real world, much like the way we choose to immerse ourselves in video games and television. Imagine if, with quantum computing, we could create perfectly realistic environments or even universes. We'd be able to control our reality and just create a sort of 'digital heaven'. 7. We could already be in one of those digital realities. This 'simulation hypothesis' was published by Nick Bostrom in 2003 (I think?) and was a very interesting idea. If true it would have profound implications. Summary in spoiler. [spoiler]"A technologically mature "posthuman" civilization would have enormous computing power. Based on this empirical fact, the simulation argument shows that at least one of the following propositions is true:The fraction of human-level civilizations that reach a posthuman stage is very close to zero;The fraction of posthuman civilizations that are interested in running ancestor-simulations is very close to zero;The fraction of all people with our kind of experiences that are living in a simulation is very close to one.If (1) is true, then we will almost certainly go extinct before reaching posthumanity. If (2) is true, then there must be a strong convergence among the courses of advanced civilizations so that virtually none contains any relatively wealthy individuals who desire to run ancestor-simulations and are free to do so. If (3) is true, then we almost certainly live in a simulation. In the dark forest of our current ignorance, it seems sensible to apportion one’s credence roughly evenly between (1), (2), and (3).Unless we are now living in a simulation, our descendants will almost certainly never run an ancestor-simulation."[/spoiler] Also I suppose I have to concede there's a very small chance they are among us.