It's a question we earthlings have been grappling with for a long time, but the answer may not be straightforward.
This question originally appeared on Quora: Hypothetically, if there is intelligent alien life, with the knowledge and means to traverse space and travel to Earth, what would be their reasons for not making contact? Answer by Javed Qadrud-Din
People frequently underestimate just how much more advanced spacefaring aliens might be. I would posit that there are races out there so much more advanced than us that we would be unable to recognize the evidence of their existence, and such evidence would instead appear to us as aspects of the nature of reality itself.
The universe is old enough that aliens might have a multi-billion year head start
The universe is 14 billion years old. There is debate about how recently enough heavy elements were fused to permit the formation of rocky planets harboring the elements required for life as we know it. Some believe that rocky planets with the ingredients for life could have formed as early as 12 billion years ago , but let's take a conservative figure of 8 billion years ago.
So planets capable of supporting life existed in the universe 8 billion years ago. Our planet formed 4.6 billion years ago. It took another billion years for the surface to cool and for conditions supporting life to arise. Almost immediately upon the conditions becoming suitable, life arose on our planet 3.6 billion years ago.
Assuming things work roughly the same way on other planets as they have here on Earth, we can reasonably conclude that there are life-harboring planets out there with a 3.4 billion year head start. This means there are planets out there with a head start almost equal to (and perhaps greater than) the entire time life has existed on Earth.
In billions of years, life has evolved such that primitive life forms are incapable of comprehending the more advanced ones
Now consider how different humans are from the first life forms to arise on our planet. Assuming the rate of evolutionary change remains constant, the life forms on a planet with a 3.4 billion year head start would be about as different from our level of complexity as we are from the simplest bacteria.
Bacteria are entirely incapable of comprehending us. In fact, they are not capable of being aware of us at all. They inhabit the same space we do, even living inside our bodies as symbionts, but they are entirely incapable of being aware of us. Consider ants, which are far more complex than bacteria and are separated from us by only 500 million years of evolutionary development. They too inhabit the same space. They can be affected by our actions. They can see us, eat the food we drop on the ground, crawl all over our picnic tables, but they aren't really aware of us. They are entirely incapable of understanding any message we might try to send to them. We have come to understand the chemical signals ants use to communicate with each other, and we can lay down trails for them to follow etc., so basically we humans are able to speak an ant's language, but still we cannot convey to them any concept of what we are because ants lack the brains to comprehend what we are.
So, assuming the pace of evolutionary development is roughly constant, aliens out there would likely be so much more advanced that we would be incapable of comprehending them even if we were living right in front of them on their picnic table. They could even learn to speak our language, as we have with ants, and it wouldn't do much good. That in itself could explain why it seems that no alien intelligences have contacted us. But really, the difference is even more extreme than the rate biological evolutionary development would suggest.
With the advent of technology, the rate of development has increased enormously, such that the complexity gap will be even bigger over the coming few billion years
About 40,000 years ago, humans began to develop technology. Unlike evolutionary development, which advances by a process of random mutation, technological development advances through directed and systematic procedures. As a result, the pace of development has accelerated enormously.
Most people are probably familiar with cargo cults. For those who are not: cargo cults exist on some remote islands in the pacific. During WW2, the American and Japanese militaries established bases on some remote islands that had little to no previous contact with developed civilizations. When the war ended, the bases were abandoned. The natives on the islands had seen technologies that they did not understand. The lights and flying machines appeared to them as magic. Now, some of the people on these islands have adopted rituals that mimic the movements of air traffic controllers and radio operators, hoping that they can call down cargo from the sky.
These people are separated from us by only 8,000 years of development (the time it took from the establishment of the first agricultural civilizations until the present day) and yet they couldn't understand what they saw, and they worship our technology in a religious manner.
8,000 years is far less than an eye-blink on the billions-of-years timescale of the head start that some planets had on ours. It took 8,000 years for us to appear as gods to other humans – how much longer would it take before we would recede from their ability to understand altogether? Maybe 10x that amount of time? But consider also that the rate at which technology is developing is increasing exponentially. Most of the developments that separate us from hunter-gatherers were made in the last 200 years.
So, with technological development beginning 3.4 billion years ahead of ours, the gap between us and an alien race would not be just the gap between bacteria and us, but conceivably exponentially greater.
TL;DR Now consider again the ants. They can't conceive of what we are. All they know is that there are lots of crumbs that tend to show up in a certain area near their anthill. For the ants, this is just a fact of life--"there are lots of crumbs over there usually" is just part of the nature of reality as they perceive it. Similarly, the evidence of alien life is probably all around us, but they are so much more advanced than we are, that the evidence of their existence appears to simply be woven into the nature of reality as we perceive it.
 How Old are the First Planets?