10 Theories Why We Haven't Discovered Aliens Yet
5. They Aren't Anything We Recognize
In Terry Bisson's short story They're Made Out of Meat, two aliens have discovered the human race, while one tries to explain to its partner that humans are 'made of meat'. It's implied that the two aliens, and every other known species, is not made of flesh and bone but could instead be machines or any number of things incomprehensible to a human. The alien has difficulty understanding the concept, and it leaves a hollow feeling about the narrow way humans look for alien life.
The way used to search involves finding the things that humans need to survive. This makes sense, it is geared towards a human understanding of life, but it might not be the correct one. They might not require oxygen or water, and scientists don't know if the only kind of life that exists is ours, and thus have not calibrated their searches for anything else.
Aliens could only be a stone's throw away, and our definition of life could just be too narrow to include them.