Stephen Wolfram CEO of Wolfram Research and author of A New Kind Of Science discusses the Fermi Paradox: Why hasn't Earth been contacted by ET civilizations?
His answers to the Fermi Paradox:
Extraterrestrials are visiting, but we do not detect them;
Extraterrestrials have visited, but not in recorded history;
Extraterrestrials choose to exist in other dimensions;
Interstellar travel is somehow infeasible;
Colonization is somehow ecologically limited;
Physical travel is not worth it; only signals are ever sent.
Explanations for apparent lack of radio signals include:
Broadcasting is avoided for fear of conquest;
There are active efforts to prevent us being contaminated;
Extraterrestrials have no interest in communicating;
Radio is the wrong medium;
There are signals, but we do not understand them.
References: The solutions to the Fermi Paradox listed above are from the discussion on SETI in this free online book by Stephen Wolfram: A New Kind of Science.