Why haven't we met any aliens? According to Geoffrey Miller:
"Basically, I think the aliens don’t blow themselves up; they just get addicted to computer games. They forget to send radio signals or colonize space because they’re too busy with runaway consumerism and virtual-reality narcissism. They don’t need Sentinels to enslave them in a Matrix; they do it to themselves, just as we are doing today."
From Seed Magazine: http://seedmagazine.com/content/article/why_we_haven
And referred to in the engrossing End of Absence.