Seth Shostak (Space.com)
Well, it’s because the credibility of the evidence is not predicated on its complexity. If SETI were to announce that we’re not alone because it had detected a signal, it would be on the basis of artificiality. An endless, sinusoidal signal—a dead simple tone—is not complex; it’s artificial. Such a tone just doesn’t seem to be generated by natural astrophysical processes. In addition, and unlike other radio emissions produced by the cosmos, such a signal is devoid of the appendages and inefficiencies nature always seems to add—for example, DNA’s junk and redundancy.
Related: The Search for Scientific Cover—“Intelligent design’s” use of the SETI analogy