To the atheist, it doesn’t really matter. There is something, therefore something must exist. Atheism itself is not a belief system, so cannot even address the question of “why” that something exists.
Atheism, in it purest form, is actually neutral to the existence of God. The Atheist may personally object to specific beliefs regarding God, but acknowledges that God’s existence or absence would be objectively independent of his or her belief.
I am proposing that there might be an atheistic explanation for this question: Albeit, the explanation might appear to be nothing more than a play of words. The reason why there is something rather than nothing is because it is impossible for “nothing” to exist by definition.
Empirically, we have no way of knowing if “nothing” could exist, because we observe something. The atheist simply does not know, and has no way of knowing. The atheist accepts that it could be by complete happenstance that something exists, that there is no explanation as to “why” anything exists.
(I also believe that it is impossible for nothing to exist. However, my reasoning is different. I believe it is impossible because God is a necessary being and God’s essence is existence itself.)
What sayeth you?
Note: The theist is obligated to explain why God exists. Why? Because God is something, not nothing.
The theist is in the same boat. God exists, therefore God exists. The Christian God even chose this as his name “I Am Who Am” (I am the one who exists).