Why do atheists argue against a God they do not believe in?

Why not? Believing that there is no God is a reason to argue against him.  Are you suggesting patronisingly that atheists are trying to talk themselves into not believing?  It takes some arrogance to accuse an atheist of being a believer!

Why do Christians argue against atheism that they do not believe in?

We must remember that the doctrine of God insults human beings not just atheists so we are entitled to condemn it.

Belief in God is a matter of truth and it has consequences.  If it is a lie then there has to be bad consequences even if it is simply bad feelings towards atheists.

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