We could stop assuming that, but it would be a bad assumption. We have several thousand years of direct observation that the average person "groks".
The real driver of the evolution of society came when the "Love thy neighbor" requirement was instituted. It works really well, and probably explains aa lot about the accension of modern man. The best part is that it can insert itself into any political system. Unfortunately, the latest fashion is for pseudo-intelectuals to throw the baby out with the bath water and search for the meaning of life inside of black holes. Sure, I'm viewing this in a biased way, and I don't recommend turning world governance back over to the 3 idiot sons of Charlemagne, I guess I'm convinced that if you believe in unalienable rights, it requires a belief in God. I've found that most people that don't believe in God come up with overly absurd assumptions and excuses because the alternative of loving your neighbor is really hard work and imposes direct personal responsibility. This in no way infringes on my right to keep and bear arms (just in case i haven't shoe-boxed enough).