Atheism is not a religion because quite simply it falls outside the definition of a religion. Particular system of faith and worship 2. Human recognition of superhuman power and esp. of a personal God or gods entitled to obedience and worship. Atheism is based on fact, on scientific knowledge, it requires intellect, not faith. All religions have some sort of worshipping ritual, atheists do not worship.
Worship is primitive.
Religions involve belief in the supernatural, atheists reject all belief in the supernatural. Atheism is about reality, religions are about fantasy. Atheism is as much a religion as health is a disease. Atheism is science Atheist philosophy is science philosophy. Atheists know that everything that exists and happens in this universe is physics.