UFO as you know stands for unidentified flying object, not aliens.
UFO's are military related. Very highly developed technology even in the 40's. The pilots who report these sightings - even Air Force - are not privy to top secret military developments and experimentation. That's a very closed circle. Stealth tech is a good example and once made known, sure explained a lot of previous "sightings".
All the evidence points to extremely advanced military operations, which can introduce it's own conspiracy theories.
People don't seem to comprehend how monstrously huge the military industrial complex is worldwide, particularly in the US, and the enormous investment into R&D.
http://www.physicsforums.com/showthread.php?t=4842 - the Debunking Napster in the Physics Forum
http://www.csicop.org/si/show/ufos_and_aliens_in_space - "The mission of the Committee for Skeptical Inquiry is to promote scientific inquiry, critical investigation, and the use of reason in examining controversial and extraordinary claims."
Sure, we could post links pro and con "aliens are real" all day long. I prefer to stay focused on sources with objective analysis.
I do understand why people choose to believe in the alien concept. I'm just not one of them.