My personal guess is that complex live needs a long time to develop and also stable conditions during this long time- which basically rules out a lot of systems

My personal guess is that complex live needs a long time to develop and also stable conditions during this long time- which basically rules out a lot of systems with sporadic, subtile deathbringers. That plus a vicious streak to evolution, allowing for developing of fast counter measures to any sentient species. (Any bird, carrying the plague, would have the world to itself.) might account for alot of the silence.