Because "atheism" simply means a rejection of the concept of "god." It has nothing to do with evolution, gravity, warp drive, plumbing repair, software design, medicine, or baseball.
Remember Thomas Jefferson:
Jefferson never heard of the theory of evolution.
Remember Thomas Jefferson:
Quote:Question with boldness even the existence of a God; because, if there be one, he must more approve of the homage of reason, than that of blind-folded fear.
Jefferson never heard of the theory of evolution.