Is the premise correct? Is it true that Socrates and Jesus were wholly original? Jesus at least was foreshadowed by Hillel's Golden Rule.

The Carvaka had their own ethics; Epicurus had his own, almost identical, as I think you know.

"Almost always stems" is a rather weak point. Most theists don't come up with their own ethics either, and there have been rather more of them. And while the theist could make up anything based on faith, materialists seem to converge. Actually theistic ethics, as opposed to theist arbitrary rules of behavior, tend to be convergent as well; there's more diversity in the justifications put forth for ethics rather than in the ethics, I think. Nowadays for justifications or origins we get more extreme forms of utilitarianism (Singer), social justice ideas (Rawls), ideas out of evolution and game theory (Axelrod, Prisoner's Dilemma, kin selection).

But the ethics? Be good to one another. Don't lie. Or else do what you want and can get away with. Or do what you're told -- not too common among atheists. Not a whole lot of possibilities for theists or atheists.