The main reason why the elderly die most often

There are other reasons, but I think it's fractures.
If a fracture occurs anywhere in the lower body, they can't walk for several months.
Muscles atrophy. They can no longer walk.
Since families cannot care for someone who cannot walk at home, they go to nursing homes.
Once in a nursing home, they age even faster.
Then depression sets in, and they live like that until they eventually pass away from pneumonia.