Imagine there's a bridge that crosses a river. During an earthquake, it collapses and hundreds of people on the bridge die. A tragedy.
What if we could live in a world without bridges? Then this tragedy would never occur again. Those hundreds of people would live forever.
There are two ways to not have bridges. One is to do without, and the other is to invent some other way to move people across rivers. These new ways would somehow have to never lead to deaths in order to be humanistically pure.
According to humanism, human suffering and death are bad. If you agree that they are bad, raise your hand. Huh, it looks like everyone in the room has their hand raised. So everybody believes this... you can't go against it now.
Okay, so if you really believe that human suffering and death are bad, what then? How can you not make human well-being your ultimate value? (Or the avoidance of harm to humans.) What kind of person are you, if you don't? You will be asked by people around you, asked searchingly and demandingly. What kind of person are you if you don't find something precious, ultimate, first in your heart, sharply and deeply poignant to you? And what kind of person are you if you don't find human well-being to be that value? How can you possibly have friends with any but the maximally prosocial point of view? The social pressure to be the real version of some societal value causes a society to become more intense, monocultural, rigorous, emphatic, pure.
People have a thirst for reality, to be real people, to have contact with reality outside themselves. They will seek reality wherever it leads them.
No, people don't live forever. Maybe you would think that that would cause a society to let go. "Okay, people have to die sometime, maybe prolonging every life doesn't have to be our priority." But oddly enough, the awareness that people don't live forever makes us look at life as precious, and thus at every individual life as precious. So when we get around to it, at great expense, we will replace bridges with the next, safer option. Because we get so much benefit from crossing rivers, we have to have bridges. We have to have expensive bridge replacements. We have to save every life if we possibly can because we have to seek the highest, as we understand it.
So we make prosociality our god in this respect, as we do in others.
(If you found this post interesting, you can read my book Letters to People Who Care which talks about this in its own way.)