Because the U.S. still has not even come close to mastering the separation of church and state which means depending on where you live it will confront you frequently whether you like or not. Burying your head in the sand does nothing if you believe the premise of that separation is virtuous. I am not atheist myself but I have yet to see anyone I know be vocal about their atheism, yet I know many that are vocal about their religious views. I am curious where you see vocal atheists enough that they annoy you?