That's to keep ahead of Russians & Chinese.
Which has it's reasons, but can be out of place itself, because UFOs don't lend more often in one country than in another. And as soon as US announces that it is actively researching UFOs everybody else will dust off his archives.
Russians are aware of UFOs from times deep into good old Cold War. Communist party secretary Andropov, who was head of KGB on his previous job, formed so called Institute 21
( maybe number was different ) located on outskirts of Leningrad. Institute 21
(?) was staffed by 16 full time science specialist from different areas, whose only job was to centrally collect and analyse UFO data. Soviet interest in UFOs was so intense that on occasion all branches of Soviet army were issued with order to watch night sky and submit written reports if they've seen something out of usual. After Soviet collapse director of that Institute tried to sell all the case files to some US media company who made a documentary about it. Documentary can be found on YouTube.
Separate research was done by Soviet Navy. Soviet Navy had one full time researcher who was organising and analysing UFO reports coming from Soviet Navy's ships and submarines. He published a book, available on Amazon, with hundreds of encounters.
I don't know what Chinese know about UFOs, but its wast country, so they must had lots of UFO cases.
As well, since 1947 Roswell crash US gov. had material proof that UFOs were real. So, inspired by this knowledge, during the 50's US gov. literally splashed Manhattan's amounts of cash at all companies in military industrial complex. All that US gov. cared about was that these companies put in an decent effort, regardless of the outcome. All companies that worked on these UFO projects earned profit margins of between 150-200%
( source: Dr. Louis Witten, physicist who worked for Glenn Martin Co. on exactly these projects ), so they were extremely eager to take this work. Eventually, by the end of 50's in early 60's significant progress was made and first man-made UFOs were produced. These UFOs were maintained in Palmdale, CA in Lockheed Martin's workshop. Late ufologist Mark McCandlish did great research and presentation on MUFON's conference about Palmdale facilities. Essentially, when man-made UFOs would come in to land, police will cordon-off roads for miles around Lockheed's workshop and McCandlish actually interviewed policeman who managed this.
But these secrets were buried in private companies to avoid FOIA requests. Some of these UFOs, probably the latest models, are held in a base close to the Guam, in Mariana archipelago. Far enough from eyes of public, and taking into account incredible speeds of these vehicles, close enough to China & Russia. These man-made vehicles are used, among other things, to make frequent visits to the Moon and UFOs are stuffed by marines from Guam base. That info came from Dr. Greer's interview with one of marines who helped handle these UFOs. That video is still on YouTube.
Disclaimer: these are things I worked out by carefully placing together and cross-referencing various slips-of-tongue in UFO testimonials and more-or-less official documents.