Because is disagrees with their holy texts.
Most Christians work around this, saying that the creation story is allegorical, or poetic liberty, or something else that lets them ignore the huge contradiction between their "inerrant text" and reality.
Others resolve this conflict by claiming reality is wrong, because their holy text must be right. Because it says so.
The problem is that once you start saying bits of a holy text are not true, but are actually meaningless in the light of what is real - - - then where do you stop? The fact is that the huge discrepancies between the Bible stories and the historical record show that the Bible is no better than all the other texts that we now put firmly into the Folklore and Mythology section.
Generally speaking, atheists have come to terms with that fact, and have dropped their belief. The majority of Christians have not come to terms with that fact, and are in a spiral of "interpretation" and "allegory" trying to reconcile their holy text, what they want to believe and what they have accepted as real. And the minority of Christians who have actually accepted that their text contradicts what has been shown to be real - but who's faith is so important to them that they will deny reality rather than give up their faith.