This is the most compelling reason for me not to believe most conspiracy theories. The sheer number of people that would have to be involved combined with an internet age just isn't the best formula for secret-keeping, even if you do buy everyone off. You can pull up individuals that have kept secrets of this magnitude for their entire lives, but lots of people with different backgrounds and values? Nuh uh.
The closest example I can think of is Ultra in the UK, but that was pre-internet.