It seems to me that the real problem is that these issues have root causes which really need to be addressed, and that's why telling the chronically obese people "just stop eating" doesn't work, because it isn't addressing the root cause.

The problem, however, is that addressing the root cause is extremely difficult, or even impossible (for those trying to give advice), so people don't know what else to say. For the person in poverty, for instance, the root causes are numerous and systemic in that society, and pretty much impossible for anyone to fix anytime soon. So what else do you do or say?