Christianity's god says that we are not worthy without him.

[Angrboda]

The thing that gets me is that they could have gone the other way. How about Original Grace instead of Original Sin? How about telling kids they must be valuable because God loves them instead of they're valuable because God loves them?

There's no good reason for Christians not to tell their children they're kind and good and beautiful and deserve to be loved. A fair number of Christians do, so I don't buy that children being born worthless is a necessary part of their religion.

I think the answer to this lies in the function of the stories in the old testament. Unlike Christians, the Jews believed that a person's wrongdoing would be punished in this life, not in any hereafter. And if a whole people were guilty of error, the whole would be punished in this world. It's a cultural identity myth; it's purpose is to keep the people together by motivating them all to believe the same cultural stories. The Hebrews most likely were a strain of Semite intermixed with all the other breeds of humans occupying Canaan, and so stories evolved to make them feel special and obligated to care about that specialness. The carrot is a poor long term solution to keep a people united; the stick works much better. So they have a myth of oppression (Exodus) and of triumph (Joshua) and tales of bad, fallen away Jews being punished, and good, united Jews prospering. Just as in biological evolution, those Canaanite Jews who developed such myths stayed together and by staying together, continued the myths; those who didn't, weren't able to stay together as a people, even though they likely had their stories too — their stories didn't exert a force to keep them together as a culture, retelling the stories over and over again. So the ultimate answer is that, from the perspective of social psychology, myths of redemption and creating the idea that by being united you can be saved from something, those memes worked better in terms of perpetuating themselves by encouraging the hearer to a) think of his group membership as valuable (redemptive), and b) encouraged him to repeat and celebrate those stories. Other memes simply didn't succeed at keeping themselves alive in a cohesive, unified culture in the same way.