One of my other favorite's of the Fermi's Paradox Hypothesis is :
They are too busy online
"It may be that intelligent alien life forms cause their own "increasing disinterest" in the outside world.[94] Perhaps any sufficiently advanced society will develop highly engaging media and entertainment well before the capacity for advanced space travel, and that the rate of appeal of these social contrivances is destined, because of their inherent reduced complexity, to overtake any desire for complex, expensive endeavors such as space exploration and communication. Once any sufficiently advanced civilization becomes able to master its environment, and most of its physical needs are met through technology, various "social and entertainment technologies", including virtual reality, are postulated to become the primary drivers and motivations of that civilization."
This one is funny to me because it's basically saying an alien race mastered their own entertainment, technology, and communication and basically said fukk everything else and went to live in video games and online.
Human beings might fukk around and go this route if we are not carful.