It's apparently beyond the scope of their book (titled Science and the Good) to explore the reasons why this 400 year effort has failed to produce the desired moral code, but those reasons, it seems to me, are fairly obvious.
No non-theistic account of morality can explain why human beings have dignity, rights and worth. No secularized morality, which by its nature excludes ultimate accountability, can have any binding force.
It cannot impose duties or obligations, nor can it give a plausible explanation of what it means to say that a particular behavior is wrong. Nor can secular ethics give a satisfactory answer to the egoist who asks why he would be wrong to care only about his own interests and not bother himself at all with concerns about the well-being of other people.
This is why naturalism, or atheism, leads logically to moral nihilism even if most naturalists don't take the atheistic train all the way to that station.
Judeo-Christian morality, based on God's revelation to man, is the only plausible ground for a morality that does not end up in nihilism.
Only if we're made in the image of God by a Creator who loves us do we have dignity, worth and human rights.
Only if an omniscient, omnipotent, eternal, perfectly good Creator has designed us to be a certain kind of creature and wills that we behave in accord with the Manufacturer's specifications can we say that violating that will is wrong.
Only if the Creator holds us accountable for how we live are we, in fact, accountable in any meaningful sense.
Only if the Creator insists that we love our neighbor, with all that that entails, do we have moral obligations to others.
Take away the Creator, God, and all of the foregoing evanesces, like the body of the Cheshire cat in Alice. To the extent any of it persists at all it's just an arbitrary expression of personal taste or mere illusion.
This goes a long way, I think, toward explaining why our culture is in the inauspicious state it's in today. We're like astronauts floating about in space trying to decide which way is up. Our moral judgments, like the astronaut's, are purely a result of our own subjective feelings and have no purchase on anyone else. As Rousseau put it 250 years ago, "Whatever I feel is good, is good; whatever I feel is bad, is bad."
In his book The Atheist's Guide to Reality Duke philosopher Alex Rosenberg, himself an atheist, puts the matter more plainly than most. He maintains that having abandoned God, atheism leaves us with science as the only reliable source of knowledge.
This faith in the epistemic power of science is called scientism and, Rosenberg argues, it leads straight to nihilism, but most people, unlike him, balk at going that far. He gives three reasons for their hesitancy:
In a world where physics fixes all the facts, it’s hard to see how there could be room for moral facts. In a universe headed for its own heat death, there is no cosmic value to human life, your own or anyone else’s. Why bother to be good? ….Scientism, he avers, can’t avoid nihilism. We need to make the best of it. He writes:
First, nihilism can’t condemn Hitler, Stalin, Mao, Pol Pot, or those who fomented the Armenian genocide or the Rwandan one. If there is no such thing as “morally forbidden,” then what Mohamed Atta did on September 11, 2001, was not morally forbidden. Of course, it was not permitted either. But still, don’t we want to have grounds to condemn these monsters? Nihilism seems to cut that ground out from under us.
Second, if we admit to being nihilists, then people won’t trust us. We won’t be left alone when there is loose change around. We won’t be relied on to be sure small children stay out of trouble.
Third, and worst of all, if nihilism gets any traction, society will be destroyed. We will find ourselves back in Thomas Hobbes’s famous state of nature, where “the life of man is solitary, mean, nasty, brutish and short.” Surely, we don’t want to be nihilists if we can possibly avoid it. (Or at least, we don’t want the other people around us to be nihilists.) ….
To avoid the aforementioned outcomes, people have been searching for scientifically respectable justification of morality for least a century and a half. The trouble is that over the same 150 years or so, the reasons for nihilism have continued to mount.So, what's an atheist to do? The choice is either give up one's atheism or give up on morality. Rosenberg and others, strangely enough, opt for the latter.
Both the failure to find an ethics that everyone can agree on and the scientific explanation of the origin and persistence of moral norms have made nihilism more and more plausible while remaining just as unappetizing.