On 24 June 1947, pilot Kenneth Arnold reported seeing several high-speed, unidentified flying objects near Mount Rainier in Washington state. On 8 July the world was introduced to the term flying saucer by journalists who were describing Arnold's sighting and the spate of copycat sightings that followed in its wake. Interestingly, Arnold never claimed to have seen saucer-shaped objects. The objects he described were more like a boomerang or flying wing, similar to the modern B-2 bomber. The term arose because Arnold described the motion of the objects to reports as erratic, "like a saucer if you skip it across the water." Oregon journalist Bill Bequette, who first interviewed Arnold, misinterpreted this to mean the objects were saucer shaped. Bequette filed his story with the Associated Press and soon newspapers across the country were telling the tale of the "flying disks." Two weeks later, the Times of London was the first to actually use the term flying saucer. Arnold tried to correct the error, but it was too late. The idea of saucer-shaped alien craft had wormed its way into the public consciousness and subsequent "sightings" dutifully conformed to the saucer-shaped prototype of a proper alien craft.

people just called them that because that's what they looked like.