I tend to think that it was a mixture of the government trying to distract from/discredit sightings of experimental craft and displaced anxiety over the Cold War. (Sorry to be a bummer, that isn't as fun as actual aliens, carry on.)

https://www.nytimes.com/1997/08/03/us/cia-admits-government-lied-about-ufo-sightings.html

https://www.theguardian.com/film/2014/aug/14/men-in-black-ufo-sightings-mirage-makers-movie

https://www.imdb.com/title/tt2254010/