Lots of people are familiar with Fermi's Paradox. Our solar system is about a billion years younger than most similar star systems in the visible universe. So if life is common, shouldn't we see evidence? There are a few hypothetical reasons why we might not:

1. Interstellar travel might not be possible or economical.

2. Interstellar communication might not be possible or economical.

3. Harnessing near-space resources such as asteroid volatiles or solar energy might not be possible or economical.

4. All species produce more offspring than can possibly survive, including intelligent ones, and therefore consume all their resources and succumb to extinction or marginalization before having any significant impact on the interstellar environment.

5. There is a natural phenomenon, or a predatory species, that suppresses all other species in the universe that compete with them for the same resources, i.e. interstellar and space resources.

6. Species tend to evolve in such as way as to lose interest in space resources, in the same way as modern humans have lost interest in mundane aspects of nature that are beneith us cognitively and intellectually.

7. Life on earth may BE a form of interstellar colonization, having originated elsewhere.

8. Traits don't evolve in any particular order, so earth may be the only planet in the visible universe where a species evolved a large brain before another survival trait precluded it's development, such as hollow bones and wings for flight, or females with narrow hips that can burrow through narrow underground tunnels.

9. We just don't know what to look for, and therefore haven't detected it yet.