Scientists call this question the Fermi Paradox. Mathematics (especially a thing called The Drake Equation) says there SHOULD be planets out there supporting intelligent life, but we have no actual evidence that there ARE any.
IT'S A FUCKING PARADOX, RIGHT?
According to astrophysicists, there are currently TWENTY answers to the Fermi Paradox i.e. explanations for why we haven't heard from any aliens.
1. Aliens don't exist... Duh. What happened on Earth was such a random collision of a billion unique circumstances, it has not happened twice. It COULD happen at some point in the future. But so far, it just hasn't.
2. Aliens exist, but they're basically goo... Aliens are just a bunch of algae somewhere. Their existence is impossible for us to detect. Also, who gives a shit? It's goo. No one cares about goo.
3. Aliens don't have enough bars on their space phones... Intelligent aliens exist, but don't have the technology to send (or receive) messages over sufficiently large distances. So basically, Aliens are on Sprint.
4. Aliens blew themselves up already... Carl Sagan speculated that civilizations might invariably destroy themselves shortly after developing spaceflight technology, through the violence and outright warfare that such a giant change inevitably wreaks on society. So creating NASA was a terrible idea and has sown the seeds of our eventual destruction. Nice job, JFK.
5. Aliens are blowing each other up already... One bunch of aliens could be killing the other bunch of aliens who want to hang, before we get a chance to talk. Cosmologist Edward Harrison argued that intelligent species beyond a certain point of technological capability will destroy other intelligent species as they appear.
6. Aliens wanted to talk, but asteroids... You know what happened to the dinosaurs? With that asteroid strike, and then the volcanoes, and that whole business? Some scientists think that has happened to intelligent aliens before they got a chance to develop interstellar chat skillz.
7. There are Aliens, but only in other Universes... So it's a multi-verse thing. Do you understand the concept of multi-verses? Me neither. But Dr. Alan Guth of M.I.T. came up with the multi-verse solution to the Fermi Paradox so he can explain. Here's his email: email@example.com
8. Shit, we just missed the Aliens... So maybe there WERE technologically awesome Aliens around to talk to, but they went extinct (for ecological or other reasons) before we had a chance to chat. If two civilizations are separated by several thousand light-years, it's always possible that one or both cultures may become extinct before meaningful dialogue can be established. We could MAYBE solve this problem if we created what are called "Bracewell Probes" - Artificial Intelligence satellites we send out to talk to other civilizations (a.k.a. robot social media influencers for brand "Earth"). But we haven't yet because Elon Musk is a LAZY DICK.
10. They tried to call but we weren't in... So legit Aliens called us a few hundred years ago but we couldn't hear them because we were all killing each other for being the wrong kind of Christian and stuff.
11. Humans aren't listening properly... Maybe Aliens are transmitting signals that have a very high or low data rate, or employ unconventional or frequencies. Basically, Aliens are leaving interstellar Missed Connection posts all over Space Craigslist, but we don't go on Space Craigslist.
12. They have better phones... Advanced alien civilizations may evolve beyond broadcasting at all in the electromagnetic spectrum, and communicate by technologies not developed or used by humans. Some scientists have hypothesized that advanced civilizations may send neutrino signals.
13. Aliens stay in The Matrix all day... Some advanced beings may divest themselves of physical form, create massive artificial virtual environments, transfer themselves into these environments through mind uploading, and exist totally within virtual worlds, ignoring the external physical universe. Stoners.
14. Aliens are just too "alien-y"... We might underestimate just how much alien life differs from life on Earth. Aliens may be psychologically unwilling to attempt to communicate with human beings. Maybe an alien species might have a thought process orders of magnitude slower (or faster) than humans. A message broadcast by that species might well seem like random background noise to humans, and therefore go undetected.
15. Everyone is listening, no one is transmitting... The only civilization we know (i.e. Earth) does not explicitly transmit, except for a few small efforts. And even these efforts were controversial. It is not even clear WE would actually respond to a detected signal—the official policy within the SETI community is that "[no] response to a signal or other evidence of extraterrestrial intelligence should be sent until appropriate international consultations have taken place." However, given the possible impact of any reply, it may be very difficult to obtain any consensus on "Who speaks for our civilization?" and "What should we say?" Conversations with US may be bogged down in the Aliens' political arguments on the other side of the galaxy right now.
16: Earth is deliberately NOT being contacted... Basically, the Earth is being treated like a the Galapagos Islands. "Let's not contact life on Earth to allow for its natural evolution and development", say the Aliens.
17. The Truman Show hypothesis... This is a real idea. Some scientists hypothesize that our perceived universe is a simulated reality. We're being contained in a fake-ass planetarium. Aliens may have created this simulation so that the universe appears to be empty of other life. Good afternoon, good evening and goodnight.
18. It's too dangerous to communicate... Maybe prudent civilizations actively hide not only from Earth but from everyone, out of fear of other civilizations?
19. Aliens are actually here already, but undetected... I am an Alien, btw.