Why is the jumping off a building is death?

Answer 1

cause you break every bone of your body plus your head

Answer:

Like so many events there is not just one outcome of jumping off a building. Many people die from jumping from tall building but survive jumps from smaller buildings. Human survival of falls from the 17th floor of buildings has been recorded and several parachutists have survived falls from altitudes far taller than any building.

Death in falls usually occurs when the falling person hitting head on when landing or if the body lands on a hard (unyielding) surface. A unyielding surface causes a sudden deceleration. The body acts as fluid and spreads sidewards rupturing many internal organs and causing the brain to splash against the downward side of the skull.

Interestingly a significant amount of work has been done on the defenestration of cats in New York City (Defenestration is the technical term for jumping ot of windows). Cats dropping from 6 floors up or so die. Above the 9th floor they live. This is thought to be due to cats reaching a lower terminal velocity if they have time to relax on the way down.