Why would a government keep this stuff a secret? Why would ALL governments keep this a secret?
Governments keep secrets for a lot of reasons:
1. To protect an advantage. This makes sense if we’ve adapted some of this militarily.
2. To “protect” the people from fear. This is the “If they knew what was going on, we couldn’t control them.”
3. To “Protect” the status quo. If telling the secret would result in a disruption of the economy or balance of power, keeping mum would make sense. This applies to the religious implications as well as the revelation of tech knowledge that would make quantum changes in things like transport and power generation. Those things would destroy economies and nations.
4. Protect the government from the people. This is the result of telling lie after lie after lie. The truth would reveal that the government has been lying forever and exposing the lies would subject the government to embarrassment or such harsh retaliation that it would not survive. Examples would be if the truth were that the governments were dealing with the remnants of a Nazi government in exile (one of the conspiracy theories.). Another example would be if it were revealed that our government agreed to allow human harvesting for the aliens (the theme of the X files.)
5. There are no secrets. The government has no idea and has been telling the truth since 1947. (You have to present the most absurd options when laying out choices. LOL)
I think the most likely scenario is Option 1 and 4.
The government only cares about “protecting” people when it suits their needs.
6. The “aliens” do not want the governments to disclose.
I think there are many more alternative reasons.
For starters you have not addressed the issue of major corporations making a lot of money from alien technology.
Then we have the issue of governments conducting criminal acts to keep secrets—which forces them to keep doing more criminal acts to keep those secrets—in an endless coverup loop.
I also think there were patently illegal and unconstitutional (not to mention immoral) treaties between human governments and aliens.
Then we have the deeper problem of “fake history”—layers and layers of fake history. Those historical narratives are what persuade folks to be loyal to their government—and when ripped away the populace may become outright hostile.
There are more reasons as well.
This rabbit hole is very very deep.