I've postulated this once or twice on HN before: they're there, but they're not looking for us. Civilizations either self-destruct or they reach technological singularity. At that point, why explore the universe at the unbearable crawl of speed C, only to catalog endless permutations of the same old molecules, nothing to learn that you couldn't derive from first principles.

They spend their resources looking in the opposite direction: not outward, but inward. Down there, past ten or 11 dimensions of superstring theory, far beyond today's dimly seen fundament, beneath all those layers of turtles... that's the real frontier. Smaller, not bigger.

I think that alien civilizations just roll up into a ball and disappear from view, exploring the universe within.