Psychological background noise
I have a rather different take on it.
What's striking is that many of the most popular superstitions are utterly irreconcilable with Christianity. Take ghosts, for example. The standard Christian view is that the dead are immediately hauled before God, judged, and sent to either Heaven or Hell (in some variants, this happens on Judgment Day, when the dead rise from their graves after having been in a sort of suspended anima-tion). This doesn't allow for the dead to hang around particular sites on Earth for centuries in a non-corporeal but self-aware form and "haunt" people. Reincarnation presents a similar problem. According to Christianity, you live once, die once, and go to an eternal reward or punishment. You don't live a long succession of separate lives, one after another. It should be logically impossible to be a Christian and also believe in ghosts or reincarnation or several other popular superstitions.
Yet many people supposedly do. It's claimed that around 80% of the US population self-identifies as Christian, yet I've also seen claims that about half of all Americans believe in ghosts, or say they do. Can 30% of the population (or actually more, since most atheists probably aren't among the ghost-believers) really hold two beliefs that are so obviously mutually exclusive?
This bolsters the view I've long held, that Americans in general are not nearly as religious as they are generally thought to be. As Christopher Hitchens says, many people claim to be religious in surveys because it's expected of them, but this doesn't reflect reality. Many others have a religious belief so vague and sketchy as to be practically meaningless (other surveys show that huge numbers of self-proclaimed Christians flunk tests of even very basic Bible knowledge, for example). The only way a "Christian" can believe in ghosts or reincarnation is if his Christian beliefs are superficial and he has hardly ever given them much serious thought.
He probably hasn't ever given ghosts or reincarnation much serious thought either.
Yes, there is a hard core of seriously-religious people in the US -- the ones who send money to TV evangelists, buy their books, go proselytizing door-to-door, and vote for people like Huckabee in Republican primaries. But this is a minority. For most Americans, I think, both religion and popular superstitions are more like a hazy psychological background noise, picked up from family and pop culture and continuing by inertia, not much examined or considered, and having little impact on how life is lived or thought about. This may be depressing to those who would prefer every-one to adhere to strict standards of rationality, but it is a far cry from dangerous fanaticism.