It seems like a lot of elderly people who break their hip die? Why would this be?

Physical Stress. Hip fracture places severe physiological stresses on the body. Many will have surgery, which can also impose risks. Rehab is often difficult.. and weakness develops in unused muscle.. is a tragedy.
See below. An elderly person's ability to recover from an injury is not as good as a younger person. After a hip fracture, the elderly person will usually be immobilized for a while. This increases the risk of life threatening conditions like blood clots in the legs that can travel to the lungs, pneumonia, infections, pressure sores and other conditions. That is why it is so important to get people moving.