Countless lifetimes of Judeo-Christian tradition has produced a moral code based on an incredible understanding of human nature (leave "God" out of it). Much of this code is not supportable by casual observations and deductive reasoning, which is why it escapes the atheist who considers that s/he is a "good" person.
Why are lust and envy wrong? They don't hurt anyone. What is wrong with two consenting adults (married or not) having a tryst that no one will ever know about? What does it harm when I apply for a government benefit or file an insurance claim that is fraudulent? So what if I "fudge the numbers" on my tax return? The total harm to anyone is so small as to be negligible.
Atheists don't get it.