This may be why the US government doesn't support SETI since actual
contact with aliens would eliminate this cover.

Analysis of the history of the development of a technical species on
Earth indicates that the development of such species are extremely
rare.  That is, they are common on the scale of the cosmos, but rare
on the scale of galaxies.

Consider, that life as we know it cannot exist on Population 2 stars
that are devoid of metals.  Next, consider that around such stars
cellular life formed readily enough, but multi-celled life took half
the the time life existed on Earth.  Then, consider that it took
nearly the entire history of life on Earth for brains to appear.
Technology arose quickly after that.  But, then consider that
technology appears to be unstable.  Producing pollution, resource
depletion, atomic weapons, biological weapons.

Even given all these constraints there are so many stars in the
cosmos, there ought to be thousands of civilizations lasting millions
if not billions of years.  Yet, with only thousands across the cosmos,
there are likely only one civilization among 10,000s of galaxies.

All of which is consistent with our SETI findings - and makes it
unlikely that we're being visited by aliens.

This leaves time travelers - and covert ops that use UFO legends as
cover.