That can't be entirely true WM, the Invisible College is larger than you think, Diana Pasulka found there were quite a few.
But what about the ones purely employed by the USG or black projects, yes they risk losing their jobs but also scientist are an open-minded bunch, who generally abhor secrecy and control. I'm just surprised more haven't come forward. There are a huge number of them employed by the Air Force based near Wright Patterson.