The biggest misconception people have about aliens is that they are more advanced than humans. The reality is it is quite possible that most alien civilizations, if they exsist, could be at just the same level if more primative than humans are, and even if some sort of alien civilization that is so advanced that it has come to the point that it has created light-speed star ships does exsist, solar systems are divided many light years apart. So unless an advanced alien civilization living about 1000 light years away also has a life span of about 3000 years, chances are the aliens would die on the way to earth and back. That is why I don't think aliens have visited earth before.