Comment by HilbertSpace

Possible answer: First nearly every civilization that mastered, say, radio, did it millions of years ago. Second, nearly every such civilization long ago found much better means of communications than we know about. Third, they realize that trying to communicate via radio is silly and use much better means we don't know about. Net, they are out there and communicating, but we don't hear them and they don't hear us or bother with us. To hear us, they would have to be within 100 light years, and that's not far enough to cover many candidate planets.