The simplest explanation is that atheism is simpler than religious belief. You avoid having to be involved in weekly ritualistic exercise, and perhaps other more frequent rituals. You can watch that football game, or play golf, instead of having to attend church.
Others, I am sure, have made a conscious decision. They have examined Christianity, and found the evidence for its foundational principles to be lacking.
Then some have not got to the point where they feel Christianity (or Scientology, or Buddhism, etc.) even merits consideration. In the case of Christianity, the world I see does not require the existence of a creator, much less a personal god, to explain it. I first want to see something that makes me believe that there is something of a supernatural nature (poor wording, perhaps) in our universe before I go investigating it.
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