The material is classified. Its composition is classified. Its use in the weapon is classified, and the process itself is classified. But at least we know it's called FOGBANK.

It's easy to joke about, but one of the other problems is that if Government X knows that the US has found out specific facts A, B, C, and D, they can then determine that the only person who could have provided those pieces of information is Person Z.

And now Z (and maybe their entire family) get literally taken out back and shot in the head. Now you not only lose the source of intelligence, you disincentivize anyone else to provide intelligence to you.

Or they find out that it could have only been gathered by obscure technique Q, they take specific countermeasures to mitigate against Q, and there goes millions or billions of dollars of R&D.