Health is a complex subject and most people just want simplicity. The world is littered with simplistic and misleading indicators and you correctly point out that BMI is one of them. I’m a reference case: at 1.73 meters tall and weighing 82kg my BMI is too high. But my body fat is at 9%, my resting heart rate is 42 bpm, my blood pressure is 118/78, my C-reactive protein level is nearly zero, my VO2 Max is that of a 30-year old athlete (I’m 60) and my HA1C score is 4.1. But every time I need to take out Key Man life insurance I have to undergo a full medical because of that BMI score.
Yet across the population as a whole, BMI does in fact act as one indicator that has modest utility provided that it is combined with other more important indicators. Problem is, no one has time. So doctors use BMI, insurance companies use BMI, and probably my Aunt Sally uses BMI when she wants to bully my uncle John. And they’ll keep on using it for a long time to come merely because it’s simple. We are, after all, only human.