You talk about common sense and see a sea of people who are trying to lose weight (cdc says about half of adults in the US[1]) at very low success rates (<1% according to this study [2]) and think: they must be ignorant or not trying hard enough. I just don't really see how you could reasonably make that case.
1. https://www.cdc.gov/nchs/products/databriefs/db313.htm#:~:te....