I think Atheist for the most part have made the choice to "be their own God". Simply put, join the ultimate rebel against God, because of wanting "their own freedom", to do whatever they want.
I think when some do realize how pointless that life is, they seek God, but because of the fact that religion has failed society, some of those individuals resign themselves to Atheism or Agnosticism.
However, some find a religion they feel they can bond with.
When that fails them, they just decide to be non-religious, but "spiritual".
I think though, you are referring to die hard Atheist. These as I said earlier, for the most part hate God, and his authority. They want their own rules.
Salvation of Christ don't appeal to them because it does not come with freedom from authority... at least individual authority.