We're all different. Some people aren't going to be thin short of medical intervention.
I tend to eat like a 10 year old if I don't pay attention, and if I do pay attention I can drop weight fairly easily.
I'm never going to be thin. It's not my body type. My doc called me a "mesomorph" which means big bone structure, unusually large hands and feet, lots of muscle, and can gain or lose weight pretty easily.
BMI is also B.S. and not applicable to people like me.