My guess? Because it's easy, and because the patient can make changes that show up in numbers. Even if those numbers cannot distinguish between added muscle and added fat.

FWIW, there are electronic scales that can estimate body fat, water weight, and muscle mass. So on the one hand, you can look at the scale and know that, yes, that lunch at the diner WAS extra salty and you're retaining water. 😊 But there are also times you see you dropped two pounds over the weekend... and three of those were muscle. 😫