Because god isn't real and atheists are.Open to all atheists:
Atheists say they lack a belief in God.
God says that isn't true, that they do know of Him, but suppress that truth so they can continue sinning.
Now God, by His nature cannot lie.
But the atheist, appealing to subjective morality, is free to lie at will.
Therefore why would anyone believe an atheist who claims that they lack a belief in God?
Glad to help.