The same reason many people (including myself) are Atheist; religion doesn't answer enough questions.  Science holds the knowledge religion doesn't.  And because science is fascinating; finding out the secrets of life, why we exist; is there more beings, or we're simply accidents. 

It's the only way to know how the world works, religion often tries to answer it, but have and always will fall short in providing actual answers to how our universe is run and strung together. 

Those who truly wish answers must admit guess work, tradition, emotion and the fallacious argument that purpose is intrinsic to our existence is bad ways to go about. 

Inb4 butt-hurt atheist, my last two girlfriends have been catholic, and my best friend thinks Adam and Eve was our true ancestors and that the flames of hell is as real the cloudy heaven he believes in. 

I'm just saying, if you actually care to know for real... you have to stop being so self centered and accept science and reason.