A simple question deserves a simple answer: I am an atheist because I have no reason to be otherwise.
I was raised religious, but even at a young age, it didn’t “take”. I accepted what I was told as truth, but I didn’t believe what I was told. I’m not actually terribly good about believing in things, which is what I appreciate about the scientific method and the natural world. What I believe is not important to the world outside of my head, but what I do is. It’s extremely liberating to not have to invest belief in things- I just accept what works and ignore what doesn’t. New evidence can’t challenge my beliefs, only change what I accept as useful.
I’m still a human being, and I still very much want certain things to be true. I can’t claim that I’m always so coldly rational, but it’s something I get to aspire to.