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Why do most scientists consider UFO reports and crop circles insufficient evidence of visiting aliens?

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artist's conception of Kepler-186f — extraterrestrial intelligence An artist's conception depicts Kepler-186f, the first Earth-sized extrasolar planet found in its star's habitable zone.

Most scientists consider UFO reports and crop circles insufficient evidence of visiting aliens because these phenomena lack scientific verification and often have terrestrial explanations.

Regarding crop circles, while some proponents argue they are messages from extraterrestrial life, many have been proven to be human-made hoaxes created with simple tools like ropes and boards. Even those who view them as unexplained phenomena acknowledge that a significant portion are the work of humans.12

Similarly, investigations into UFOs—such as the U.S. Air Force's Project Blue Book and the Condon Report—found no evidence of extraterrestrial origin. Most sightings are attributed to natural phenomena, such as Venus, atmospheric conditions, or optical illusions, while others result from secret military tests. Psychologists also attribute many "abduction" reports to sleep paralysis. Because these reports are often ambiguous or easily explained by human activity and natural occurrences, they do not constitute the rigorous, direct evidence required by the scientific community to confirm the existence of extraterrestrial intelligence.345