Since when is psychology not part of biology?

Certainly, if you eat enough less and move enough more, you'll lose weight. I'm pretty sure there's a minimum threshold to this.

The problem with this is "eat less and move more" is just physics and not really a complete picture of your physiology. That is - you can be impelled somehow to eat less and move and if you go over threshold, you will lose weight. The problem is you may not reach a state which your body's internal calibration systems consider normal and thus you will wind-up gaining the weight back when the forces impelling you to eat less and move more go away. There's more to a craving for food than "psychology".

This is where the difficult biology of weight stands, as I understand it.

And personally, I'm always underweight no matter my food consumption so it's not hard to imagine other in the opposite situation. Each person's physiological system is different so a thousand antidotes don't prove a generality although they can certainly show the exceptions to the supposed "rule".