It may be a fundamental interaction of our social instincts and our greater intelligence. One can over rationalize to the detriment of society on occasion; it helps to have some built-in check to that.
I tend to avoid formal statements on things like morality. Formalisms always seem to leave out important elements.
But I do have a personal opinion on how morality evolves.
I think that morality arises more from empathy than from a calculus of self interest. Empathy is perfectly consistent with rational self interest, and operates in the absence of analysis and great intellect. It is also amenable to being codified, which is what religions do. When morality gets codified it is subject to rational analysis.
I think that in any given age, the brightest people tend to support codified morality and rational analysis. That's their strength and the path of their self interest.
Codes of behavior always fall short of perfection. There are always bright people looking for loopholes. So there is always tension between those who support the codes and those who attempt to subvert them. And there are always reformers.
Now suppose we have a reformer who says the codes are OK, but irrelevant. All the codes are based on the principle that you must act from empathy. You must put yourself in other people's shoes and make their interests your own.
Of course this is just the meandering thoughts of an infidel.