I've been thinking about it, and more and more it seems to me that this is part of the thought process that allows people to believe some wacky things.
I think that in general, we look at the world in terms of mental models. We all like to have accurate mental models, so we try to verify them. When it comes to creationism, a person could go through their whole life without encountering anything that disagreed with the model. Mainly, this is because the issues of creationism don't come up very often in the lives of most people: whether you believe in a young Earth or an old Earth, 99.9...% of the concerns in your life are going to be exactly the same. It's only if you work, say, as a geologist or a biologist (or frequent forums like this one) that you'd have creationist beliefs challenged on any sort of regular basis.
Also, creationism doesn't ever come in isolation. It's always a subset of some larger set of beliefs about God, the universe and our place in it. Because, as I mentioned, the details of creationism only actually impact a person's life in very minor ways, this means that even if you score a few points in the creationism vs. evolution debate, the creationist is still able to look at their beliefs as a whole and realize that even with that "damage" to, say, their ideas about the age of the Earth, their larger mental model still agrees with what they observe 99% of the time or more, which is still pretty accurate as models go.