Suggested reading: Rare Earth: Why Complex Life is Uncommon in the Universe by Peter D. Ward and Donald E. Brownlee. The basic premise is that complex life is incredibly rare, because only through a series of effectively chance events, did complex life happen on Earth.

Even if each of those events were actually inevitable, planet size, radiation received from host star, exact chemical makeup of atmosphere/oceans and any number of other variables could affect the time scale for a planet to fruit.

But on the universes timescale, a slight variance could still be thousands of years or more. So maybe there is life in some distant reaches of the universe, who are +/- a thousand years from our level of development. Their signals simply haven't reached us yet, or ours them.