Does it make sense to define atheism as "a lack of belief in a God" rather than as "a belief in the nonexistence of God?"
Every atheist lacks belief in a God; but lacking belief in a God is not sufficient to make one an atheist. Consider babies, squirrels, or stones. They lack this belief (and perhaps all beliefs) but it seems rather odd to describe them as atheists. We presumably want to reserve this term for those who have considered the arguments and evidence on both sides of the issue.