If it helps, I think you may be assuming that all these space agencies knew that the earth was flat, and therefore knew that every claim of space exploration was a lie. That almost certainly wasn't the case back then. Nowadays, I'm sure they realize that prolonged space travel is impossible (and possibly even that the earth is flat, although I personally doubt that they'd have the inclination or scientific rigor to figure it out) and that other space agencies must be lying too, but back in the fifties and sixties, for all they knew, they were the only ones who couldn't properly explore space.