On topic, there's another theory of why we haven't seen evidence of aliens, and it's been lurking around the edges of this discussion a couple of times.
Life is competitive, and, at any given level of technology, the universe is finite to the user of that technology. Hence, available resources are finite for any given level of technology (effectively speaking). Why, then, do we assume that any other intelligent life is willing to share, especially if the rate of technology progress is not uniform of doesn't have discontinuities. Let me explain that last -- our technological expansion from muscle power to atomic power was, at first, extremely slow (we used muscle power for the vast history of our species, with slow progress in technological improvement). The Chinese developed technologies that were crucial to the technological boom (gunpowder and the printing press) but did nothing with them for centuries. Then, suddenly, we exploded. We may stall again, possibly soon. There may be points ahead where we cannot see past where we are (light as an unbreachable speed limit could be reality or just a roadblock we're not seeing around -- we can't say). A different species may have a different experience with technology altogether, and realizing this we cannot assume that any alien species we find will not suddenly leapfrog past us in technology.
So, to recap, other species are direct competitors for a finite resource pool. They may also realize that technological capability can be in fits and starts, and a given alien race may rapidly surpass their ability to compete. Finally, intelligence is, by far, most commonly developed in predator species (or omnivore species) here on Earth. That may hold elsewhere. All of this means that the best survival tactic for aliens is to not advertise their position to possibly more powerful competitors.
Add to the above the lack of communication that is likely. Completely different anatomies along with different cultural and social structures means that communication with an alien species is likely to be very difficult. Misunderstandings are likely. But, most importantly, there's a trust factor. Can you trust the alien, who may suddenly and quickly surpass your ability to defend yourself? See any number of uncontacted tribes on Earth for examples of this. In this low trust environment, staying quiet and striking first and hard are rational approaches. We may actually be in a reasonably populated region of the galaxy, but all of our neighbors are survivalist loners who plan to shoot claimjumpers and never ask questions.
Now, I'm not sure I buy the above. But, I can't discount the possibility, either. Nor can I claim credit for it -- that goes to Liu Cixen and his novels Three Body Problem and The Dark Forest. The above is developed and fleshed out in the latter. It's an interesting possibility. I do think it does a better job than usual at attempting an original solve to the Drake equation with a reasonable set of inferences. I also like that it addresses the unfounded optimism many display at the idea of friendly aliens. Basic interaction among humans should depose that idea. Also, the stark statements that technological advancement isn't guaranteed and can stagnate despite best efforts. If there's a key understanding that we lack, we will not be able to move past it. Nor may other intelligences.
