And their management procedures for controlling
Why would the government keep these things secret? There are several possible reasons that come to mind, any one of which would be an excellent reason:
* They are unable to reliably control our airspace and knowledge of this would make citizens feel unsafe.
* UFOs and extraterrestrial visitors are real and they fear knowledge of this would have an adverse effect on our economy, world religions, or result in public panic.
* Treaties or alliances with one or more extraterrestrial civilizations have been signed without Congressional approval.
* We have successfully reverse engineered these craft years ago, discovering new principles in science and have created our own secret space program.