TWO REASONS! It's hiding and/or it's not biological. Meaning it evolved beyond the biological to the technological. In other words. Aliens are machines. AI machines. Undetectable, whether deliberately undetectable by our current technology, or situationally undetectable meaning they are so highly advanced they do not produce any evidence of their being, i.e. waste heat. A Type III civilization would be capable of using all the available energy in a galaxy. Meaning, likely, no waste heat detectible. It could also be that advanced civilizations are deliberately hiding. (Lookup Dark Forest Hypothesis). I think it's both. Biological life evolves into technological life, then into quantum life. (see my Quantum Life Hypothesis which posits there are 3 stages of evolution; 1. biological, 2. technological, 3. quantum). Humans are looking for signs of biological life and of radio signals; primitive tech and primitive life compared to the cosmic scale of the age of the universe. In other words. We're looking for the wrong kind of life and our tech is not advanced enough to detect the more advanced life out there! We can't even detect life or evidence of life on Mars yet and it's right next door. We surely don't have the technology to detect technology more advanced than what we have much less machine silicon-based technological life. We should be looking for evidence of machines, AI, self replicating machines. Perhaps something like Von Neumann Machines. Not biosignatures and radio signals. That is what we should be looking for!