>Question: are there any other known "Manhattan projects" and if not, why not?

Answering this question is necessarily an exercise in tautology. If there were any other extant Manhattan Projects, we would not citably known about it because, being a ‘Manhattan Project’, it would be replete with secrecy, including disinformation to keep the public from giving leaks about it any credibility. (case example, the Alamogordo Trinity test was called an ammunition explosion accident, by way of an Army Air Forces colonel, for the benefit of the good people of nearby Albuquerque).

Many today suspect the UAP study subject as being an ongoing decades-long Manhattan Project, from the late 1940s onward. As Congressional hearings leading to just this past Tuesday’s, Sep 9th have amply demonstrated, there increasingly appears to be some veracity to these claims. https://m.youtube.com/watch?v=LBKRr5OvF6E&t=1056