Atheism is an ism
From Greek: A is without; theos is god. Atheism is not "I don't know," that's agnosticism (again, Greek: A is still without; gnostos is knowledge). Atheists affirmatively say there is not a god (or God, depending on your opinions about capitalization, or gods if you are Hindu). It is one among the many isms that proclaims to affirmatively know the answer and, as such, bears the burden of proving that answer is the correct "one" among the, literally, infinite number of possible answers.
Not much different from saying "there is one answer and it is the Flying Spaghetti Monster" in my book.