The Fermi Principle stems from a question posed by Enrico Fermi, "if there is intelligent life out there, why have we not encountered it yet?" He goes on to say that a species that could master space ravel could quickly expand and conquer the galaxy.
That whole comment is just wrong on multiple levels.
No single human civilization has survived more than a few hundred years. The belief is, of course, that previous civilizations did not possesses the intellectual sophistication to expand beyond their narrow scope and therefor collapsed, whereas ours possesses a longevity theirs did not. A concept that is mind numbingly arrogant. Many of those civilizations would not look upon ours with wonder, anymore than we do theirs. We aren't that special. But to assume that once you've reached a certain level of technological sophistication…which we have not, just visit any common port a john…that you suddenly transcend history and can never suffer the degradations of all civilizations is just wrong.
Like any living organism Civilizations collapse for a variety of reasons, but age is a huge part of it. Populations decline with wealth acquisition…the biological drive to secure oneself through numbers is dampened with ample food supplies…and when populations decline or stagnate the civilization begins to suffer a wide range of other problems.
To assume that this concept doesn't apply to other species that evolve in similar environments is a pretty big assumption. So alien civilizations could have grown, expanded and collapsed any number of times without us being privy to them. We've only been poking around for about 4000 years, only in the last 500 have we had telescopes begin enough to see beyond our solar system.
Furthermore, that intelligent life might not be bound to our own terrestrial needs. Perhaps some creatures from a gas planet have evolved and expanded through the galaxy and seeing our own planet can do nothing with it. No resources for their use.
The idea that we can determine what life is like elsewhere because we see a slight wobble in a flicker of star indicating a planet, doesn't mean we can see that planet. Hell, we can't even see Venus through the cloud bank; nor even the other side of our own moon unless we go there!
With this in mind Fermi's question is too narrow minded and bound by terrestrial habits.
Why haven't we seen extraterrestrial life? Maybe, just maybe, because we don't know what to look for and when we do see it, we don't believe it.