Why does using a measure of body weight not tell us everything about how healthya person might be?

Because a person's health depends on a number of factors besides the measurement of body weight.

Explanation:

Being healthy is much more than good measures. Some analyzes of a person's health are only feasible through laboratory tests such as glucose level, triglycerides, cholesterol. A person may be of the right weight, have a balanced diet and still have diabetes, for example. This is because a person's health depends on many factors, such as biochemical, physical, psychological, etc.

Body weight consists of what we call fat mass and lean mass. Fat mass is the fat we have in the body. Lean mass is the joint formed by our muscles, organs, bones, blood and other body fluids.