Fact is, we really don't know what the conditions for life are so saying 'well only so many planets orbiting certain kinds of stars can produce it' is fallacious. Certainly, we can assume that our conditions, when replicated, could produce similarly advanced civilizations, but that doesn't mean they WILL and it doesn't mean civilizations could arise through other conditions.
Life is an incredible rarity if it exists outside Earth. So far it seems only one of our nine planets supports ANY life, and it has taken millions of years for that life to reach the point where it was even able to form the mental concept of extraterrestrial life.
The universe is like an enormous ocean and we are in a tiny boat in a storm, trying to hear the sound of someone else shouting.
ZZ, this gets to the core of my original post. Nearly everyone comes from the angle of we dont know what conditions life can exist in or it would take hundreds if not thousands of years for our signals to reach another intelligence and for them to reply. I used to be of this latter hypothesis myself until I started properly researching the subject.
The test for intelligent life existing in the galaxy is not one of can we hear their signals as SETI has been doing for years. There is a far simpler method. And that is simple observation not of radio signals, but one for artefacts or objects.
The speed of light is a law of physics. FTL travel is not possible. If it were, we'd have evidence by now of not only other species within this galaxy travelling here there and everywhere, we'd also see evidence of travellers from outside our galaxy ie Andromeda and beyond. Wormhole travel is fine in theory but again, if it were possible, we'd see evidence of wormholes opening and closing in the galaxy. But we dont.
So we can confidently say there is no species which has the technology to travel faster than light nor create wormhole travel.
Our Milky Way galaxy is some 10+ billion years old. There are an estimated 200 - 400 billions stars in the galaxy. The majority of these are not even stars similar to our own. But even if they were, statistically at least, with the age of the galaxy, some intelligent species should have arisen. Some will have anihilated themselves, some may have been anihilated by a dinosaur killing asteroid.
If any had survived to be technically superior to ourselves, they would at least attempt to
harness the power of their sun ie build a dyson sphere
explore the galaxy in their own spacecraft or
spent out exploratory probes ie von Neumann probes or Bracewell probes.
If the galaxy is teeming with life and any such intelligence had of pre existed us by just one million years, we should easily be able to observe a structure such as a dyson sphere. In fact, if the galaxy is teeming with intelligent life older than us, we should see many, many dyson spheres in every corner of the galaxy. But we dont see a single one.
If the galaxy is teeming with older intelligent life, we should have been visited many many times by these explorers. Some people assume we dont see such evidence because they want to keep us at arms length because of our aggressive insular tendencies. But, if the galaxy is teeming with life, it only takes ONE lifeform to make itself known to us. And even if amongst all this teeming older intelligence, would they all be peaceful? There's a 50 \ 50 chance some would be aggressive expansionists so keeping us at arms length wouldnt be an issue for them. But we dont see a single instance of an older intelligent lifeform.
If the galaxy was teeming with older intelligent life, we should find plenty of evidence of von Neumann or Bracewell probes. But we havent observed a single such probe.
Taken all this empirical evidence (plus the lack of a single accepted SETI candidate signal despite 40 years of looking), we can conjecture we dont see any such evidence or aliens or alien artefacts because they dont exist right now.
If they dont exist right now, that means we are the oldest most technologically advance lifeform in the galaxy right now and there's no one out there.