The physicist Enrico Fermi wondered why we haven’t seen any evidence of visitors from another planet, given that he believed intelligent life elsewhere in our galaxy was highly probable. (Maybe we have seen such evidence, given some recent UFO incidents, but for the sake of argument…) This question is known as Fermi’s Paradox.
Standard answers to the Paradox involve emphasizing the vast distances involved, and the fact that “as far as our galaxy is concerned, we are living somewhere in the sticks, far removed from the metropolitan area of the galactic center,” as Edward Teller put it. Another theory is that species which are sufficiently intelligent to achieve interstellar travel have a tendency to blow themselves up long before they reach anywhere in our vicinity.
Don Sensing cited another possible explanation, suggested by Geoffrey Miller:
I suggest a different, even darker solution to the Paradox. Basically, I think the aliens don’t blow themselves up; they just get addicted to computer games. They forget to send radio signals or colonize space because they’re too busy with runaway consumerism and virtual-reality narcissism. They don’t need Sentinels to enslave them in a Matrix; they do it to themselves, just as we are doing today. Once they turn inwards to chase their shiny pennies of pleasure, they lose the cosmic plot. They become like a self-stimulating rat, pressing a bar to deliver electricity to its brain’s ventral tegmental area, which stimulates its nucleus accumbens to release dopamine, which feels…ever so good.
See my post here for thoughts related to the above explanation and the psychology of decadence.
But I have a new theory, suggested by recent events: The aliens invent something like Twitter, their whole planet becomes the equivalent of a particularly nasty middle school on earth, and they melt down under waves of mutual accusations and denunciations.