...because they insist that morals must come from their deity.
They absolve themselves of association of any guilt or crime due to their acting within the limits that their interpretive books tells them is okay - murder, mass murder, genocide, incest, stoning, blood sacrifice, beating, rape.
I'd rather sit with the atheists, their morals don't depend on an invisible sky-daddy with a dodgy temper, and they have no angry face in the sky to kowtow to.
They know that killing, rape, murder and theft (and so on) are wrong, because it harms somebody else. They know it's wrong because they have reasoned that it must be wrong.
The religious person has no morals of his or her own - they are all imposed, and sometimes the deity demands that a believer do things no sane, rational person would do, and yet the believer will without question.
many religious people say "hah, humanist morals - you can never get to an ought from an is!" but...if you say your god is the creator of the universe, therefore you ought to obey him, aren't you doing the same thing?