back in the day people used to see the stealh bombers and not know what the fuck it was and call it a ufo.
a lot of sightings has been documented yrs later as military planes ect but for the conspiracy nuts its a hard pill to swallow.
sure there are other life forms in the universe no doubt, have they visted earth, probably (thats as far as im going - the rest of you can argue about the validity of being kidnapped by aliens or meeting them
) but most of what people see are military projects.