Why are atheists obsessed with the Bible?
Simply put, they are not. Many atheists are not particularly interested in religion at all, similar to how many religious people are not particularly occupied with unbelief.
However, you will find that most people who take anti-religious positions in English do so against Christianity. This is simply due to the fact that most English-speaking countries have a majority (or at least large proportion) of Christians as the religious makeup. The involvement of the Bible logically follows.
Keep in mind that atheism involves a non-belief in *all* religions, but all-encompassing knowledge is impractical. I for instance am no expert at all on the Aztec religious beliefs yet I do not believe them. My ignorance is reasonable given the lack of impact on my daily life. On the other hand I was raised as a Christian and my parents are both believers, so my relatively in-depth knowledge of the Bible is to be expected.