This is because creationist's beliefs did not arise from careful consideration and logical thought, they came from indoctrination and/or personal revelation.
The belief exists, so whatever is written in the book is true. If the book is true then whatever impossible myth under discussion must have come about some way. It doesn't matter much how, because it's true and any explanation is fine - even the daftest.
It's an alien way of working things out to me and I'm baffled by the self delusion and often simple dishonesty of it and often wonder how people who think like this deal with life. I think the thought processes of a fundie deserves a thread of its own.
Life, don't talk to me about life - Marvin the Paranoid Android
When you’re trudging through the pouring rain to the office, it seems like the Earth possesses an infinite amount of water, a not-insignificant amount of which is dripping down your collar. But when you see an image like this one, produced by the USGS, it hammers home the reality of the situation: the water’s all spread out in a very thin layer, like a millimeter of frosting on a cake. If you gathered all the world’s water—from oceans, lakes, groundwater, water vapor, everything—into a sphere, it would have a diameter of 860 miles. That’s the distance between Salt Lake City and Topeka, Kansas.
That’s still a fairly big sphere, when you think about it: that same water spread out in an even layer across the United States would leave us under a 90-mile-deep lake. But it isn’t nearly as big as you might expect, looking at our blue marble in photos from space or dipping your toes in the Atlantic. To boot, very little of that water—less than 4%—is freshwater, and the vast majority of that is locked up in glaciers and ice caps. We’ve got just a tiny fraction of that sphere at our disposal; it behooves us to use it wisely.