re: Why do fit people not wear Fitbits?

because if you're workout is primarily cardio, you will give up before you reach your goals. It doesn't burn fat as effectively. You are only burning while your heart rate is increased. The instant you stop, levels are back to normal.
When lifting weights, you are breaking down muscle fibers, which takes up to 3 days to repair and rebuild. During this 3 days, you have dramatically increased protein synthesis which in turns boost metabolic rate. During this time, your body is using energy to rebuild what you have damaged. If you are calorie deficient, your body will then use stored energy (fat) to repair the damage.