I believe that the most likely reason that we’re not seeing any evidence for the existence of aliens is that none of them have developed the necessary civilization. That’s even though I think it’s likely that there are many independently evolved instances of biochemical life, most of them unicellular.
Engineer Ian Brownlie, writing on Centauri Dreams, has a similar view, but his is subtler than mine and he is more articulate in expressing it:
It is not intelligence itself [that is lacking on extraterrestrial worlds]. It is not tool use, language, or even culture. Nor is it the first appearance of science-like inquiry. It is something stranger and more fragile: the sustained decision to look outward, to ask questions about things that may offer no immediate survival advantage, and to keep asking them long enough for the answers to accumulate into a durable scientific tradition.
The quest for the stars requires a drive to look outwards, over many generations. Because this isn’t something evolution or any other force would optimize for in the long run, this drive is fragile, and this may be why we haven’t observed it outside of humanity.
Nehaveigur 