There are two kinds of people who argue that the Bible is meant to be taken literally:
1. Bible Literalists
2. People who dislike Christianity and who are determined to make it seem as stupid as possible.
Both are problematic in their own way, but here I'm only going to address the second group.
There are different kinds of atheists, and the kind of atheist that is most disposed toward biblical literalism is the kind that isn't just atheist, but who is outright hostile towards Christianity as a whole, based on the actions and interpretations of certain Christians that the atheist has encountered and heard.
In short, a person who is prejudiced against Christianity.
In order for Christianity itself to be the buggaboo that he believes it to be, then any Christian who doesn't follow a strict, fundamentalist asshole interpretation of the Bible must be doing it wrong.
As one of the many, many Christians who is not a strict fundamentalist asshole, I take issue with this kind of prejudice, and will step up and call people on this BS every time somebody tries to lump all Christians in with the fundamentalists.