For ages humans have been wondering if we are alone in the universe or not, and tried to find any kind of life outside our planet but nothing has been found yet.
Here are my top 3 reasons why we haven't found aliens:
The immensity of the universe
The distance beetwen the solar system and it's closest star (Proxima Centauri) is 4.24 light years away, that means that if we could reach somehow the speed of light, it would take more than four years for us to reach the star.
If any race of aliens lived there, first they would need to find some type of transportation which could make them move near the speed of light, and then they should have an objective or reason for visiting us, because they wouldn't travel that distance for pleasure.That doesn't seem that difficult for a really technologically advanced but it's improbable that there's alien life that near of us.
If we wanted to find aliens they should live in our galaxy, because the biggest problem with distance begins when we talk about the distance beetwen galaxies, even the distance beetwen the milky way and andromeda the closest galaxy is about 2.537.000 light years, so if they traveled here even at the speed of light they would probably die before reaching us.
Most civilisations destroy themselves
The commonly known “great filter” hypothesis is that at some point in the development of intelligent forms of life there is often some obstacle that stops it's further advancement. It could be anything, like war, scarcity or even global warming,
This obstacle or obstacles would end up with a large number of alien civilisiations and only a tiny part would remain, so life in the universe is abundant but it dies after evolving much, so there are only some advanced civilisiations that could contact with us.
In the case of humans, some people think that Extinction is built into our biology to ensure that we are eliminated before we create too much of an imbalance on Earth, and make sure other life can evolve and develop without us destroying them.
We are emitting more and more co2 every time, (as more developed more co2 emissions) so if we don't reach a high level of technollogy soon we will destroy ourselves too.
We Can’t Recognize Their Signals
This option is for me the most possible, because if we can't even undersand some animals sounds, and it's difficult and takes a lot of time to understand and learn a non native language it would be impossible for us to communicate with an alien civilisation with only signals.
We could have already found them but we just don’t recognize their presence. The problem is that we’re looking for something very similar to us, assuming that they at least have something like the same mathematics and technology when they probably don't.
It would be especially difficult when trying to communicate with a highly advanced civilisation because they might use untraditional communication methods (such as neutrinos or gravitational waves) beyond our current technological understanding.
