In 1950 Enrico Fermi asked this question, more accurately he asked, “Where is everybody?” The universe is so old and large, it has been argued, that there has been plenty of time for many technologically advanced civilizations to arise and spread across the galaxy. Yet, there is no evidence of such life. Interestingly, it is this lack of evidence that is the most compelling argument against there being any other life in the universe. Enrico Fermi may have been one of the smartest people ever to live but I didn't let that discourage me from coming up with my own reasons.
More specifically I decided see how many reasons I could think of to explain why we haven't been visited by Aliens.
1.) They already did and we didn't notice. Many People believe Earth was visited by Aliens in the distant past. Sadly there is no 'conclusive' proof.
2.) There are no Aliens. It is entirely possible that we are the only examples of intelligent life in the universe. Whoopee, the Universe is all ours.
3.) The Aliens haven't found us yet. The universe could be so vast that other intelligent life just hasn't encountered us. Or conversely intelligent life could be extremely rare.
4.) They have found us, they want to surprise us and they're on the way. This one is popular in Science Fiction. We can't get to them so they gotta come to us. I hope they're friendly.
5.) Intelligent life could be self limiting. The competitive instincts of evolution causes intelligent beings to destroy each other in warfare. This one appears all to likely. Let's hope our wisdom can out pace our technology. Artificial Intelligence before nanotechnology please!
6.) They are already here but they are robots in disguise, or lizards, or something. Too weird, I let you Google this one.
7.) Aliens are deliberately avoiding contact. They may be avoiding us for our own good. Sort of like Star Fleets prime directive in Star Trek. We know form Earths history that when an advanced culture comes in contact with a more primitive one, the primitive one usually suffers. Either its culture morphs to take on the characteristics of the more advanced one or it is destroyed trying to compete. Along with the civilization that spawned it.
8.) Aliens just don't have anything to say to us. Another likely scenario. Any aliens that have existed longer than us – pick your own time frame - would be more technologically advanced. They may have enhanced their intelligence to such an extent, that they would view the universe and their place in it much differently than we do. We just may not have much in common. Humans don't spend much time talking to moss and insects and from an Alien perspective that may be just what we look like. Growing and crawling on the surface of this planet. Let's just hope they don't poke a stick in our nest.
There are many other subtle variations on these ideas but I think I got the obvious ones.