When it comes to the existence of extraterrestrial life, there are always those who ask the question: "If they exist, why have not they contacted us yet?" There are at least five answers to this question.
Our human consciousness can not allow even the thought that representatives of other civilizations that can live not only on other planets in other Galaxies, but even in other Universes, are not like us. Most scientists, drawing conclusions about the possibility of having a life on this or that planet, rely on our earthly life, and what if they are completely different? What if we walk every day by extraterrestrial beings, but do not even notice them?
In 1973 there was a sensational hypothesis - the Zoo hypothesis. Its essence boils down to the fact that higher civilizations have known all about us for a long time, they are watching us, but they simply do not interfere. We are just as unreasonable for them as the animals in the zoo for us, we keep the animals in a cage, and we are kept on the planet Earth.
Based on the second reason, we can conclude that the aliens with us just bored. People go to the zoo for half an hour and an hour, and then it gets bored. The next visit we make a few months later, or even years. So the aliens, they come with excursions, show us, but they just do not want to contact us, just as we do not even get into a cage with a monkey.
There is also a variant of the development of events, like living in different time periods. For example, there was life on Mars, but they died out a few million years ago, and now we are trying to find their traces of being on the Red Planet. Also we, after some time, may die out, and representatives of other "humanity" will look for traces of intelligent life on the planet Earth.
Well, the last option. We have always been and will be alone in the vast infinite universe, since the birth of life is so unique and fantastic that it has never happened anywhere else in the vast expanses of space and will never happen. The most sad version, but apparently that's exactly what those who ask the question about aliens think: "If they are, why have not they contacted us yet?"