Because he understands the Fermi Paradox.
Or more accurately, he understands the implications of the Fermi Paradox and The Great Silence.
Because the Galaxy appears uncolonized and unperturbed by intelligent life, and because there has been ample time and motive for this to happen, we have to conclude that some kind of filter is in place that prevents life from arriving at this advanced phase.
In his recent article for Technology Review, Bostrom writes:
...the evolutionary path to life-forms capable of space colonization leads through a "Great Filter," which can be thought of as a probability barrier...The filter consists of one or more evolutionary transitions or steps that must be traversed at great odds in order for an Earth-like planet to produce a civilization capable of exploring distant solar systems. You start with billions and billions of potential germination points for life, and you end up with a sum total of zero extraterrestrial civilizations that we can observe. The Great Filter must therefore be sufficiently powerful--which is to say, passing the critical points must be sufficiently improbable--that even with many billions of rolls of the dice, one ends up with nothing: no aliens, no spacecraft, no signals. At least, none that we can detect in our neck of the woods.We are hoping that the filter resides in our past, that we have already overcome highly improbable odds.
Now, just where might this Great Filter be located? There are two possibilities: It might be behind us, somewhere in our distant past. Or it might be ahead of us, somewhere in the decades, centuries, or millennia to come.
More disturbingly, however, it's likely that the Great Filter still awaits us in the future. There's some kind technologically instigated event that exists out there -- and no species can avoid it.
Again, Bostrom writes:
Throughout history, great civilizations on Earth have imploded--the Roman Empire, the Mayan civilization that once flourished in Central America, and many others. However, the kind of societal collapse that merely delays the eventual emergence of a space-colonizing civilization by a few hundred or a few thousand years would not explain why no such civilization has visited us from another planet. A thousand years may seem a long time to an individual, but in this context it's a sneeze. There are probably planets that are billions of years older than Earth. Any intelligent species on those planets would have had ample time to recover from repeated social or ecological collapses. Even if they failed a thousand times before they succeeded, they still could have arrived here hundreds of millions of years ago.Bostrom, who is the director of the Future of Humanity Institute at the University of Oxford, concludes his article by making a case for increased foresight and vigorous inquiry into potential risks.
The Great Filter, then, would have to be something more dramatic than run-of-the mill societal collapse: it would have to be a terminal global cataclysm, an existential catastrophe. An existential risk is one that threatens to annihilate intelligent life or permanently and drastically curtail its potential for future development. In our own case, we can identify a number of potential existential risks: a nuclear war fought with arms stockpiles much larger than today's (perhaps resulting from future arms races); a genetically engineered superbug; environmental disaster; an asteroid impact; wars or terrorist acts committed with powerful future weapons; superintelligent general artificial intelligence with destructive goals; or high-energy physics experiments. These are just some of the existential risks that have been discussed in the literature, and considering that many of these have been proposed only in recent decades, it is plausible to assume that there are further existential risks we have not yet thought of.
But even so, Bostrom asks, what makes us think we'd be immune to such a powerful filter?
Which is why, when he looks up at the stars, he is thankful that we have yet to see any signs of extraterrestrial life.
Read the entire article, "Where are They?"