UFO sightings - merely mass hysteria!
During the 1890s, there were rumors that an American inventor had perfected the first heavier-than-air flying machine. All of a sudden, tens of thousands of people began claiming to have seen the flying machine. They saw it over cities; they saw it over oceans; they saw it over rivers and deserts, and every other place one can imagine. Some claimed it had wings that flapped like a bird. Others saw giant fans and a big light on it. However, there was only one certainty, the ship didn't exist yet.
The rumors had become the flash point of the current term "mass hysteria". These sightings continued through the 20th century, until present times. The reported 1947 Roswell, New Mexico UFO crash served as a big booster shot for UFO sightings. It also created very fertile ground for the immensely popular weekly Superman series that began at theaters everywhere.
People's appetites for strange visitors from other planets became so exaggerated that abductions of earthlings became sort of common in newspapers, paperback novels and films. Most of these ET's must have come from the same planet because mostly they are described as being filmy white with large heads, line-drawn mouths and large bug-like eyes. Oh yes, they are always reported to be far ahead of us.
People can only wonder if there would have been any sightings had there been no early rumors of an American inventor with a new flying machine. The fact the government agencies will hardly ever confirm anything reported to them only lends more spice to the subject, because most people don't really trust government anyway? Do you?