I realize I didn't answer your question about why would governments conceal it.

It makes a lot of sense for our governments to conceal technology to have an edge against adversaries should a major conflict arise. The element of surprise is very powerful, and suddenly deploying technology that an adversary doesn't know how to/doesn't have an existing plan to counter is highly valuable.

Concealing technological developments also makes it more challenging for adversaries to replicate our tech.

The reason for the historical concealment from the public goes back to the Robertson Panel, who decided that while UFO/UAP represent no direct threat, they believed that public awareness could lead to ontological crises for the highly religious country at that time. They also believed that it served as a vulnerability that the Soviets could use to instill mass panic (think of the War of the Worlds radio broadcast panic).

So the Robertson Panel recommended concealing the information on UFO/UAP and initiating a 'public information campaign' to reeducate our population and to stigmatize the subject. They were very successful and this stigma still remains, as exemplified in the users who downvote factual information because they'd rather engage in confirmation bias and denial.

Everyone can read the declassified report on the Robertson Panel yourselves too. This is all verifiable, valid declassified government records.