Why do they try to make the fact the aliens existed a hidden secret?

While it's obvious as the sun. Not only are they salvaging their artifacts and concepts but also making their identity hidden from the public.

Is it just to keep it away from those who can use it in bad ways? what if it's discovered by the evil hand in the first place?

how come aliens are being taken as a joke and not a serious threat, maybe they're not a threat as a species but the things they left us in the world may lead to our doom.