That's because they don't have any muscle.  First, 29% of subjects classified as normal weight and 80% of individuals classified as overweight according to BMI had a body fat percentage within the obese range. Thus, on an individual basis BMI tends to consistently underestimate a person’s adiposity. This data implies that there are many individuals who don’t weigh that much on an absolute scale, but a large proportion of their weight is composed of fat tissue.