Most learned people have no doubt. It's just that the aliens are thousands of light years away. And most of them are algae.
This is madness! How can anybody have "no doubt" about that? The most we can say with any credibility is that there is a small chance they have come (in a way we cannot detect).
This is of course true. But the bigger problem is that "the US Government" is not a monolith. It is composed of greedy selfish untrustworthy people who will do anything to get re-elected. The two major parties cannot even control their renegades like MTG or AOC, much less conspire with the other party to hide a big story for so long.