Also, when people assert that UFOs are probably just an optical illusion, they are (or at least should be) doing that based on the long observational baseline of many investigated incidents which indeed turn out to be optical illusions. That is to say, it's legitimate to say that perceived UFOs are probably optical illusions it's because there's real data demonstrating that perceived UFOs really are optical illusions.
The illegitimate version of that would be saying it based on general arrogance, which is frequently a barrier to scientific progress. See, e.g., the story of Nobel Prize winner Barry Marshall.