> The actual physical mechanism is simple
Sure. Running faster is also just increasing the power to weight ratio.
You are technically right, but nobody doubts the high level physics of it, it’s the biology part that is complex.
Even leaving aside socio-economic factors and pure behavior issues, it’s not just a “throw things in and you can predict what goes out”. Knowing how different bodies manage food intake, food processing and retention, then come up with a calory burning budget is damn hard. We’re not pipes, a lot of things happen in the body.
I know it’s so much more comfortable to think it’s simple, but I think we need the courage to accept it’s not as straightforward as motivational poster would want us to believe.
Educating people, modifying their behavior, and increasing their willpower and persistence is what's difficult - that's a sociological problem and I agree, a hard one. The actual physical mechanism is simple.