Actual religions have sets of ideas (rules, creation-myths, alternative epistemologies, etc.), whereas atheism has none. The term literally means "without god(s)" (which was a defining feature of religions by those who coined the term).
It is an absence of belief (at least in the way the term is commonly used; There are "atheistic religions" in the sense that they don't have a deity).
Atheism is to religion as being silent is to speaking – and you're essentially claiming it's just another way of speaking.