People sometimes forget that U.F.O. stands for Unidentified Flying Object and there are way more possible things that an unidentified flying object could be other than an alien spaceship. The above link has extensive explanations. Here are some:
That some UFO cases are simply invented by witnesses is readily proven by documented research and confessions, but it is very difficult to argue more broadly for such an explanation. Indeed, studies show that less than one in 100 cases is hoaxed – although with alien contact reports and photographic cases that percentage rises sharply. The greater the kudos and attention, the more likely a case is to be a hoax.
After investigation, analysts agree that between 90 and 95 per cent of all UFO sightings prove to be Identified Flying Objects. Over 300 different things have been misperceived as UFOs – including a bin bag, a shaggy dog and a telegraph pole. The Null Hypotheses proposes that all of the remaining unsolved cases would become IFOs given enough study and sufficient evidence. However, statistical analysis (like that conducted by the Battelle Institute and French aerospace researchers GEPAN) has indicated differences between solved and unsolved cases that challenge this proposal.
Psychologist Carl Jung wrote an early treatise on UFOs in 1959 and saw hints that the consistent types of sighting being made matched deep-seated mythological archetypes within the human mind. The theory suggests that the disc shape favoured by UFO reports or the tall, blonde aliens and small dwarf-like beings seen by witnesses everywhere match strong subconscious motifs that power our psychosocial needs. UFOs could be a space age guise for the same phenomenon that has fuelled cross-cultural belief in myths and legends for thousands of years.
via the BB