The doylist explanation is that a lot of religions back in the day practiced animal sacrifice to their deities (including judaism, e.g. Noah sacrificing animals after the flood and Abraham sacrificing a ram in place of his son once god was bored of telling Abraham to kill his kid to prove his faith). Jesus getting sacrificed is supposed to be a mirror of this for Christians and an “ultimate” sacrifice. They don’t sacrifice animals to god anymore because jesus just keeps doing the heavy lifting for them.