It may a combination of A) a very big universe, and B) rare conditions for complex life.
If you take the universe and eliminate all the galaxies that are unsuitable for the formation of life, all the stars that are too young or too short-lived for intelligent life to evolve, all the stars that exist in uninhabitable parts of galaxies where there is too much radiation and too many collisions, and the regions of galaxies that have been hit with radiation from supernovas and such, then you’re left with a smaller ‘inhabitable zone’. Of that, eliminate the stars that didn’t evolve planetary systems, the ones that never wound up with a terrestrial type planet in the habitable region around the star, and you have even fewer.
Of those stars left that can support life, you need to have a habitable planet that evolved complex life, and managed to avoid being hit by a life-ending collision for four billion years. We have no idea how unlikely this is. Perhaps it’s a one-in-a-trillion shot. Perhaps we exist in one of the least-dusty lanes in our galaxy and got very, very likely. Perhaps you need a huge giant like Jupiter in your system to act as a cosmic vacuum cleaner.
Of all the rest that can support complex life, how many wound up without magnetic fields, or with some poor combination of elements or gases, or never developed a climate feedback system to keep temperatures stable, etc? Of the ones that look like Earth, how many developed intelligent life? There may be millions of planets with the equivalent of fish and snakes and deer, but perhaps intelligence it just extremely rare. On Earth, of the billions of species that have ccome and gone, only one became intelligent enough to develop technology. Maybe that was a tremendous fluke.
Of the planets that developed intelligent creatures, how many were driven to develop technology? Perhaps that’s rare as well. If Dolphins were highly intelligent, would they start building machines, or would they gear their intelligence towards singing sonnets to their mates? Perhaps if Humans had had no predators and no need to shelter themselves from the environment or harvest food, we would have evolved our intelligence very differently.
Then of the ones that are left and developing technology, how many survive to our point without wiping themselves out or constantly setting themselves back technologically through war, famine, and disease?
At every step along this chain, we are really clueless about what the numbers are, by orders of magnitude. This is really my problem with ‘scientific’ efforts to determine how many civilizations there are. You can build all the wonderful formulas you want, but if you’re populating them with wild guesses you’re not achieving anything. The formulas are useful in that they help direct our quest for answers, but at this point we can’t even begin to use them to come up with anything resembling reasonable estimates.
