I think the the ribs are not a solid sheet because they need to be able to expand so the lungs can get an adequate amount of air in the lungs. The brain just needs to be protected and doesn’t need to expand so it’s a solid sheet. The skull seems to be the better protector at first because the ribs can break and puncture a lung, harming your breathing but if you do hit your head your brain can start swelling and the skull doesn’t allow any room for expansion. Head injuries are much more dangerous, especially if the brain starts swelling. Rib injuries seem much simpler to fix and less dangerous.
