Several governments may be in possession of NHI crafts. Despite our greatest efforts, consider that a NHI may not want us to have these technologies.

What I mean is, if I were an advanced NHI observing humanity with all their flaws, nukes, and wars, maybe you would not want to enable them with your own technology. As a result, I can imagine all of these crafts (some of which are purportedly intact) could have failsafe mechanisms to prevent us using it.