I’m not certain why we’ve seen no evidence of alien civilizations. One good guess is that the speed of light really can’t be broken, so to have a civilization out there means very slow transit. But if that were true, why isn’t the sky lit up with old radio messages? Or maybe there’s just something better than radio. I don’t believe that technology runs away in every case and destroys intelligent life. But perhaps there’s some kind of technological threshhold before interstellar spaceflight which constitutes a transcendence, as in Vernor Vinge’s A Fire Upon the Deep and other far-flung sf epics.
I do think that there’s at least a chance that a lifeform bent on destroying all competition is the dominant entity in the galactic neighborhood. Cheery thought, but wouldn’t they be the most likely to survive? In a very short time, we’ve dominated and subverted every other lifeform on our planet. And we’re not even evil. Heh.
Enjoy nature while you can, kids