Space is big. Really big. You just won’t believe how vastly hugely mind-bogglingly big it is.
Pondering this fact, in 1950 Enrico Fermi over lunch posed the now famous question concerning extraterrestrial life: "Where is everybody?" He was right: Considering the size and age of the universe we should be up to our eyeballs in aliens. Where the hell are they?
So far, SETI (Search for ExtraTerrestrial Intelligence) has been a total bust: a complete and utter waste of time and money. And that's great!
As Robin Hanson pointed out in the mid 90's: If they would ever encounter an extraterrestrial transmission, we would be (statistically and royally) screwed, considering that we could hear a civilization approximately on our own level of development, but there is nobody more advanced visiting us.