Do you know how long light travels through the universe? You constantly look into an ancient past when you look at the skies at night. Many of the suns (stars) you see at night are probably already dead. You only see some old pictures of them.
Just think about the long travel it needs for an alien to visit us. It would be very hard to enter this solar system just in time to meet the human race in it's very short moment of existence.
Even if there are tons of aliens out there, it is not very likely that they meet us in time.
