Basically the same reason. And yes, there are some verses and/or chapters of the Bible they say are metaphorical or allegorical, but they get to pick and choose which those are. And it is usually the ones that make some other verse or chapter contradictory. But for the creation story, it is the first thing told about in the Bible. It is the first chapter of the first book, Genesis. For many Christians, if not most of them, it is necessary that the creation story as told in Genesis be the true and accurate account of how their God set everything into motion. It simple has to be that way. There's no reason for it other than it has to be as it is written in Genesis.
If they start accepting that Genesis is, in fact, not true as written, then they feel their entire faith system comes crumbling down as it would be possible to remove God from the equation. And they just can't have that happen.