I think many of those reporting sightings may be sincere. I have seen things in the sky which I could not identify, however I did not assume them to be UFO's, merely some atmospheric phenomenon.
Similarly, I've been to Marfa, Texas, and watched the famous "Marfa Lights" which drift back and forth across the desert at night. They've been reported for years, I've seen them myself, no one knows what they are. However, just because they can't be identified doesn't automatically make them of extraterrestrial origin.