It follows from Christian theology (imago dei) but that's increasingly losing ground in the West. If we're just agglomerations of atoms why should I believe that all human agglomerations are equally precious?
[1] https://en.wikipedia.org/wiki/Fact%E2%80%93value_distinction
If we're just agglomerations of atoms, why would any agglomeration of atoms, human or not, be precious at all?
(I mean that less flippantly than it reads.)