I don't know why. One of the founders of AA was atheist. He just realized that acting like a bigger entity exist helped him, so he started using it as a tool.

This does not mean that he really believed that.

I had seen several atheist and agnostics go to AA without any problems. When you have a serious problem like this that makes you loose your wife, your kids, your job, you try anything that could get you out.

When you are desperate you try anything, anything, including visiting shamans.

From my point of view the people that have problems with AA will be the hyper rational people that could not test something without knowing why they are doing that.

The problem is that when you are under the influence of drugs, your mind is not really free to think, but is strongly biased(cognition bias), so what looks from inside like common sense is not objectively. You can rationalize anything.

What the AA method does is you accepting that the solution is outside you, accepting external influence without judging. This is a necessary step, at least while the influence is very strong.