• Fermi famously asked "Where are they?" referring to the fact that we haven't heard from any alien civilization but statistics shows that we should have
• See Where are they? They self-destroy like us
• Before they self-destroy, sentient civilizations want to destroy all other sentient civilizations or at least avoid being destroyed by them
• Life's first priority is to remain alive
• When life meets another form of life, especially a "smart" one, there are two possibilities: 1. one of the two feels that the other one is harmless and therefore turns it into its pet (that's what we do to cats); 2. one of the two feels that the other one is lethal and then develops the science to destroy it (that's what we do to organisms that spread lethal diseases, and that's what we did for centuries to lions and tigers)
• A civilization has to destroy the other civilizations that are capable of detroying it before they get a chance. It cannot afford the chance that it will get destroyed while it tries to assess the actual threat
• You cannot take the chance that an armed invader will kill you, especially if you cannot speak his language, and therefore you will kill him as soon as you can
• Even if you can speak his language, and he tells you that his intentions are peaceful, you have no way of knowing whether he means his words, and what kind of weapons he has (i.e. if you stand a chance of deterring him with your own weapons)
• The chances that two civilizations raised in different planets can communicate and negotiate peacefully is virtually nil
• A sentient civilization has only two options: 1. exterminate other sentient civilizations; or 2. hide from other sentient civilizations
• The reason we hear nothing from extraterrestrial civilizations could be that a safe way to avoid extermination from us is to remain perpetually hidden from us, undetected by us as well as by any other civilization
• We are not intelligent enough yet to realize that it is in our interest NOT to make contact with other sentient civilizations: they will have no way of knowing whether our intentions are truly peaceful and what kind of weapons of mass destruction we have, hence their motivation to exterminate us will be very high.
• The threat doesn't even have to be deliberate. Earthly life is a threat to non-Earthly life just because it is different and therefore most likely lethal, just like European germs were lethal to Southamerican peoples. If there is just one single plant, most likely there is an entire ecosystem, teeming with thousands if not millions of life species, most of which are probably lethal to the invading civilization. Life that already exists here (e.g. us) is lethal to any other form of life that doesn't have an immune system capable of defending from that native life Life that evolved on other planets would be right to be scared of our life simply because they don't have an immune system that can deal with the diseases spread by our form of life.
• Life kills even when it doesn't mean to. Life is, by its very nature, a weapon of mass destruction.
• Colonization of distant living planets makes no sense. If life already exists on a planet, it is most likely lethal even without the intention of being lethal. It would be a total waste of time and resources to try to colonize a planet that already has some kind of life.
• It only makes sense to colonize planets where life is impossible.
• Other civilizations may be smarter than us and have already realized that colonizing another planet where life evolved is pointless.
• That leaves only one reason to make contact with civilizations of other planets: curiosity (also known as "scientific research"). However, on this planet we seem to be the only species that is "curious": other species are perfectly happy to live when their ancestors have lived for thousands of years and to lead the same lifestyles that they led. If only one out of millions of Earthly species developed a passion for curiosity, what are the chances that species evolving on other planets develop the same passion?
• Conclusion: making contact with other civilizations risks self-destruction, colonizing other planets makes sense only on planets that are not alive, and not every civilization evolves the passion for curiosity. No wonder therefore that we haven't heard from "them". Where are they? They are too wise to make contact with other forms of life, too intelligent to colonize other forms of life, and too practical to do it just out of curiosity.

Note: several scholars think that humans have explored too little of our neighborhood and it's too early to claim that we haven't found any traces of alien intelligence, notably Jill Tarter at SETI Institute in 2010 and, more scientifically, Jason Wright, Shubham Kanodia and Emily Lubar (Center for Exoplanets and Habitable Worlds at Pennsylvania State University) in 2019 ("How Much SETI Has Been Done? Finding Needles in the n-Dimensional Cosmic Haystack").