Because good without God doesn't exist. In other words, every act is always selfish. If God comes into your life, this changes and acts and people become less selfish. Now, does that mean there are no good acts? No. People DO do good things. But most people do good deeds to make themselves feel better about themselves or they don't know why they do good deeds. It just feels good and right to them. But that's God working in them.