Why not? Because there's no such being! Is that a difficult concept to fathom? God is another interesting fairy tale - he sits along side Santa Claus, the tooth fairy, Hercules, gnomes, woodland fairies, and the bogey man. He's kind of an asshole, too. All those plagues and the smitings... not cool.
And another thing about atheists - they don't all belong to one big group somewhere. They are not defined by their non-belief in gods any more than everyone over the age of seven is defined by their non-belief of faires or Santa Claus. Of course, you catch a lot more flack for not believing in this particular imaginary sky friend.
And, tho I am an atheist, I love my life. I love my friends, my child, my family. I am in awe of this amazing, beautiful world - the sky, the clouds, the trees, the lakes, the majesty and beauty of this planet we live on. I'm hopeful. I believe that people are good. I have faith in that. I have faith in you - dear friend - faith that you will love me, respect me, and come through when I need you.
So, you can accuse me of not believing in the "big peeping tom in the sky" - but never accuse me of lacking life, love or humanity. Those things are not tied to dear old JC and company.