Has no-one heard of a force field which prevents all types of mechanical and electronic devices from recording. Its plain that any society who has mastered space travel would surely have mastered this simple way of avoiding capture of their know-how by any possible means.
Certainly us feeble-minded earthlings should not be allowed such knowledge and those that fly UFOs are undoubtedly, through their time-travel abilities, aware of this. They need only look back on our history to know what a lot of no-hopers we are.
Hence we're simply not allowed proof of what some "lucky" folk get to see.
Yeah! All for a good yarn.
Robert