Consider the number of billion to one coincidences that have got the Earth and us to where we are.
The planet is at exactly the right distance from the right kind of star.
Early in the planets existence we gained a very large satellite.
This has given us a tilted, stable rotation.
Thus there is liquid water. There are seasons. There are marginal conditions because we have tides and weather. This has not only allowed life to start, but the changing marginal conditions have allowed and encouraged evolution.
Along the way, there have been various planetary catastrophes that have rearranged evolution by changing conditions radically.
Humans, as such, have only been around for the last million or so years out of the two and a half or so billion since lifeforms existed. We have had radio communication for just over 100 of those several million years.
So, if you figure that a radio signal takes about 100,000 years to cross just our galaxy, and if someone over there notices it and replies by return, it would be between 50 and 200,000 years before the reply turned up, depending on distance, so the chances of anybody noticing anybody else are slender, bearing in mind that we don't know how long we can keep our present technological age going. If chance has allowed another planet with life forms capable of compatible technology to exist, what are the chances of ours and theirs existing in a matching time frame that would allow any form of actual communication?
Yep, a lot of that complies with the rare earth hypothesis. But again Malcolm, you are coming from the angle of our signal reaching a civilisation out there and them replying to us.
However, when you start researching further it becomes clear the test of finding intelligent lifeform isnt one of listening for their signal but looking for things such as Dyson spheres, von Nuemann and Bracewell probes that an intelligent lifeform would create.
Given the age of the Galaxy, approximately 10 billion years, it can be seen life and therefore intelligent life, should have appeared somewhere in the Galaxy long before it appeared here on Earth. Earth is roughly 4.5 billion years old. Intelligent life appeared here in the last 500,000 years or so. If this timescale is transferred to the start of the Galaxy, intelligence should have appeared long before the earth was formed ie over 4.5 billion years ago. That intelligence would also be more advanced than us given the timescales.
In that time even if there was only one intelligent lifeform, it could colonise the entire galaxy even travelling at sub light speed in approximately 50 million years tops. They could send out von nuemann or bracewell probes if they didnt want to go themselves. We've already sent a kind of von Nuemann probe out in the Pioneer and Voyager crafts.
We dont see evidence of Dyson Spheres, von Nuemann or Bracewell probes, evidence to confirm other intelligence. We dont hear or see their signals, either radio or light signals that probably wouldnt be directed to us anyway.
We dont see or hear them because there's no one sending them.