Belief isn't a question of choice. Belief is fairly subconscious, and unaffected by the will. One can pretend to believe something for long enough for it to change, but it isn't a simple decision like changing political parties.

Many atheists did not "choose" to be atheists. They admitted they were atheists. Assuming they weren't life-long atheists, they just stopped believing because they were no longer convinced by the religious claims they were fed for most their lives.

It's like spending your childhood thinking that the Teenage Mutant Ninja Turtles were the most awesome thing ever, and over time, you slowly grow out of it until one day you realize that you think it's actually corny and uninteresting.

Many atheists become so, because it's important to them to be able to verify that what they believed to be true, is actually true. They care that they're accurate. Thus, all beliefs and presumptions are called into question, and many beliefs don't survive the process.