Why are hip fractures in seniors so frequently deadly?

I mean, I know of older people who get hip replacements and they seem to do fine. I know an older lady who’s had both hips AND both knees replaced, and these have not deterred her from going backpacking in the mountains of Argentina!
Yet, a surprisingly high percentage of hip fractures are said to lead to death in seniors.
So, what’s so different about recovery from a hip fracture vs. recovery from a hip replacement operation?