Sometimes people have the need to believe in some force more powerful than what they experience in everyday life. They believe in 'miracles' or 'God's vengeance'. In a more modern world there still appears to be a psychological need to believe in such nonsense, but instead of assigning such powers to God, they are assigned to 'The Government', 'Zionist Control', 'Subversives'.

In many cases these conspiracies are fueled by governments or political ideologies that find such explanations fit better with their pre-conceived view of Government, Communists, Jews, or The Enemy Within. But I think we should regard such beliefs as the atheist equivalent of miracles - there is probably a more reasonable explanation that makes more sense.

Not that I would dismiss all 'conspiracies', but 9/11, 7/7, Apollo 11 are pretty clear cut with plenty of evidence to support the mainstream view.