To spook enemies of the US and create a mystical aura of superiority around the US military and its capabilities. They're literally creating the impression that the US military has technology beyond the "Clarke threshold." (The Clarke threshold refers to Arthur C. Clarke's maxim that "any sufficiently advanced technology is indistinguishable from magic.")
To shake out moles, spies, and unreliable people / leakers.
As a cover for very terrestrial extremely advanced technology such as hypersonic weapons, drones, laser and directed energy weapons, etc.
To repeatedly float the idea of "alien disclosure" and then dash everyone's hopes in order to discredit the idea of extraterrestrial life and visitation. Why? Maybe they're concerned that we will eventually detect aliens and they want to blunt the cultural impact by making the public skeptical. Maybe they actually do have evidence of aliens (but maybe less sexy than crashed UFO parts) and want to keep it quiet, so they want the topic to be discredited and marginalized. Maybe there's some ideology at play like religious fundamentalism. Why knows.