Starting on p754 of The Cosmic Perspective, by Bennett et al, 5th edition.:
That is by far the most likely possibility. The book earlier sketched the enormous difficulties to interstellar travel imposed by the laws of physics. But in this section it just blithely assumes it will be possible some day. I suspect I know why. Frequently a history of a marvelous technology begins with a quote from some learned person in the past who said it could never happen. We implicitly condemn this person as being closed-minded and insufficiently imaginative. No scientist wants to be cited in the future in such a fashion. There are countless times scientists have claimed that something is impossible, and it in fact turns out to be impossible (faster-than-light travel, practical alchemy, perpetual motion machines), but they do not give rise to quotes that fit into historical narratives.This paradox [of why we have not met any aliens] has many possible solutions but broadly speaking we can group them into three categories:
1. We are alone.
2. Civilizations are common, but no one has colonized the galaxy. There are at least three possible reasons why this might be the case. Perhaps interstellar travel is much harder or vastly more expensive than we have guessed, and civilizations are unable to venture far from their home worlds.
If the judgment that interstellar travel is effectively impossible is correct, no other explanation is needed as to why the aliens are not here.
Continuing:
Perhaps the desire to explore is unusual, and other societies either never leave their home star systems or stop exploring before they've colonized much of the galaxy. Most ominously, perhaps many civilizations have arisen, but they have all destroyed themselves before achieving the ability to colonize the stars.
...
[This] category of solutions has ... terrifying implications. If thousands of civilizations before us have all failed to achieve interstellar travel on a large scale, what hope do we have? Unless we somehow think differently than all other civilizations, this solution says that we will never go far in space. Because we have always explored when the opportunity arose, this solution almost inevitably leads to the conclusion that failure will come about because we destroy ourselves.
The two alternative explanations lack the aura of inevitability. If there are millions of civilizations, the highly unlikely isn't going to account for the failure of all of them -- only the impossible will do. Why would we think that self-destruction is inevitable? Even if there are strong forces leading towards that outcome, there are plenty of opportunities for things to be slightly different: some variation in intelligent species' psychology or sociology, or local ecologies, or local geology. It's hard for me to see why a desire to explore would peter out most of the time, and far harder to believe it would happen in all cases.
We don't have any realistic hope of colonizing the galaxy. But there is room to hope for a long future right here on earth. We might in one way or another come up with a civilization in balance with nature, probably with fewer people than we have now. It might embody a high standard of living, social justice, and intellectual and artistic accomplishments which constitute progress over what we have today. Perhaps such a civilization would view an attempt at interstellar colonization as we today view an attempt to jump across the English Channel carrying bricks (http://montypython.50webs.com/scripts/Series_1/67.htm.) Maybe a harmonious and pleasant future stretching millions of years into the future is incredibly unlikely. But the absence of aliens doesn't figure in that discussion.
Perhaps we should interpret "If thousands of civilizations before us have all failed to achieve interstellar travel on a large scale, what hope do we have?" differently. Perhaps the author assumes interstellar travel is so important that failure to achieve it is equivalent to catastrophic failure of the entire civilization. That is a very narrow set of values that would be foreign to most people.
The absence of aliens or evidence of past alien visits implies that we will not engage in exponential colonization. That's all.
3. There is a galactic civilization, but it has not yet revealed its existence to us.
That is an intriguing possibility. A common theme in arguments about other planets, life forms, and civilizations is that what happens on any planet has no effect on what happens on any other one. We have millions of independent observations, as a scientist might put it. Once we assume easy space travel, the independence assumption disappears. The dominant one can influence events on other planets, and the galactic civilization could in statistical terms be a single data point. If its values call for not revealing itself, maybe that is why we aren't detecting aliens.
But as mundane and uninspiring as it may be, my educated guess is that you simply can't (economically) get there from here.
