When I looked into some of the ancient astronauts stuff a few weeks ago, I came across a lot of discussions of the Fermi paradox. I think I’ve come up with a possible explanation that I haven’t seen before.
The argument made by Enrico Fermi was that while it would take millions of years for a technologically advanced civilization to explore the entire Milky Way galaxy, this is not very long compared to the billions of years that the galaxy has existed. Therefore, if a technologically advanced civilization existed in the Milky Way galaxy, then they should be here by now.
A number of suggestions have been put forward to explain why we see no evidence of extraterrestrial civilizations. This range from, we are the first and only civilization to they are hiding from us. You can read about some more proposals on the Wikipedia page: https://en.wikipedia.org/wiki/Fermi_paradox
A few days ago I came up with another explanation. Some time ago I came across an article that suggested that there could well be planets that are more suited to life than Earth (see: http://news.nationalgeographic.com/news/2014/01/140117-exoplanets-superhabitable-planets-space-astronomy-science/). I thought, if extraterrestrial civilizations arose on a planet more suited to life than Earth, they might very well look at Earth as uninhabitable and pass it by.
Our space programs have sent probes to planets that we don’t think are inhabitable. But, would a civilization that had visited many solar systems still want to explore uninhabited planets? I suspect that if we explored other solar systems, that after a period where we did explore uninhabitable planets, we would come to see that as a waste of time. I’ve made the assumption that life on an Earth-like planet is sufficiently rare that an extraterrestrial civilization is unlikely to find a case before they give up on Earth-like planets.
An extraterrestrial civilization could have explored many Earth-like planets and never found one that was inhabited. If they saw Earth from a nearby solar system, they might well conclude that there wasn’t any point to actually visit. They might think: if you’ve seen one Earth, you’ve seen them all.
The main objection I see to this idea is that it assumes that the civilization had a low level of curiosity. For a civilization that had decided to undertake interstellar travel, this seems unlikely. I’m not sure we can discard the idea based on this objection, especially since, as of now, we can’t prove it one way or another.