Is a Muslim sinful for not losing weight despite knowing that obesity, lack of exercise, and unhealthy eating lead to death?

Unhealthy eating is not forbidden unless its severe and unbearable harm to an average person is confirmed.

As for obesity, what is blameworthy is that which results from indulgence in worldly pleasures and desires. However, that which is innate is not blameworthy. Jurists have differed on the ruling of eating beyond satiety, ranging between disliked (makruh) and forbidden (haram).

Regarding exercising to lose weight, it falls under the category of seeking medical treatment (tadaawi), which is not obligatory in principle except in exceptional cases. The majority of scholars have deemed it permissible or even recommended, while Hanbalis hold that refraining from it is better as it is closer to reliance on Allah (tawakkul).