Here's a better question, why do people believe "Theism" is a religion? : )
Atheism, nor theism, is a religion. The only meaning behind these words is to determine whether someone believes in a higher entity.
I don't know why, or rather how, people come to think of atheism as a religion. Perhaps it's because these people see religion as a set of beliefs, and if one does not believe in God, then that's a belief in and of itself. The problem with this logic is that religion isn't merely a set of beliefs. Religion is more specific than that, it has to do with believing in something divine in which our morality is dictated, or which nature is explained through divine explanations.
Though atheists can technically be religious, they often aren't. And I believe being a theist, you're automatically going to have a religion or non-specific religious views. This is where another common misconception arises, the belief that theism is a religion. Theism isn't a religion, as I stated before, it just so happens that you need to hold religious views to be a theist.
For example, you also need to be human (as far as we currently know) to be an atheist or a theist, but that does not mean atheism and theism's definition includes "to be human", it just so happens you need to be human to be an atheist or theist.
