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

Using a measure of body weight does not tell us everything about how healthy a person might be because this type of body mass studies do not take into account that a person's weight is not necessarily composed of fat or other elements presumably harmful to their health, that is, they do not take into account of what kind of body weight is, as it can also be composed of muscle mass and bone mass. Then, for example, a person who weighs 100 kg but its main component is muscle will not be a fat person, but on the contrary, a very healthy person with an enviable physical condition.