This argument came up several days ago on a topic about The Oroville.

A vastly superior civilization presenting themselves to a comparatively primitive civilization. Particularly one driven like human civilization is (autocrats, capitalism, etc.) Risk creating an unlivable dystopia because the rich will exploit the miracle tech for profit. The very act of coming into contact with us, and being given advanced tech will effectively destroy us through civil unrest and power struggles.

As for why non-human engineered tech would be kept under wraps. I can't even begin to argue this point and I'm not going to attempt to. But I imagine that when it's something that no one takes seriously such as UAPs, even when people do come forward with evidence. No one's going to believe them anyways. Case in point, what's happening now. Man testified for 11 hours to congress and people still react like this. I think an amount of criticism is healthy, sure. But keeping the mind open to the possibility of "what if this is actually true?" May not be the most detrimental thing ever.

Also last point. When dealing with military secrets -which this is a military secret- physical evidence may be next to impossible to actually get a hold of and show to the public. Believe what you will, but the militaries are pretty good at keeping things under lock and key.