I think the major difference here is exactly what you point out: social conditions outside of prison. The US's general lack of a social safety net, and the "ev

I think the major difference here is exactly what you point out: social conditions outside of prison. The US's general lack of a social safety net, and the "every person for themselves" attitude, means that life for a lot of Americans, in the world's richest country, is super miserable and hopeless. If US prisons treated people decently, I think many people would choose life in prison over their existing life outside of prison. Guaranteed meals, shelter, medical care, all paid for by the state? That's a lot more than many Americans have today. So for their social structures to have at least some paper-thin justifications, life in prison has to be worse than life outside prison. I am sure this is at least somewhat by design.