Remember that not only physical distance matters in this, that is, the distance from Earth to the nearest world capable of supporting not just life, but intelligent life. It is also a factor of time.
The Earth is estimated to be about 4.5 billion years old. The age of the oldest star in the Milky Way galaxy is about 13 billion years. Life on Earth has existed for about 3.7 billion years, but intelligent life, for less than a few hundred thousand.
And most important of all, intelligent life capable of communicating with another world, for a mere 75 years.
Now granted, for whatever reason, life on Earth was almost wiped out several times, and had to pretty much begin again at a primitive level.
Even if our intelligent species is able to communicate with other worlds and lasts 1 MILLION years, what are the odds that our *time* will intersect with another intelligent species able to communicate with other worlds, even if it lasts 1 MILLION years? Tiny.
Then multiply that time with those great distances found in the galaxy.
Perhaps 1 billion years ago, such a species existed just 1,000 light years away? But it doesn’t matter, because they are long extinct.
Perhaps 50,000 light years away there is another intelligent species right now. It doesn’t matter, because they are too far away. Even if they were looking for intelligent life to communicate with, we are just in one tiny spot, in a galaxy 100,000 light years across, and 1,000 light years thick.
The only way around this problem is a “profusion of intelligent life” theory. But using our planet as a model, that is highly unlikely. In 4.5 billion years, and 3.7 billion years of life, Earth has produced only one known intelligent species. In a profusion of intelligent life theory, it should have produced tens of thousands of intelligent species.
Can't really agree with this. There have been multiple "intelligent" species running around at the same time - homo sapien and neanderthals.
Also, it can be argued that there is really only room on a planet for one "intelligent" species for any length of time.