"Ultimately it's up to individuals to solve their own problems." Sure, but the problem is the externalities. We don't say 'it's up to factories to solve their

"Ultimately it's up to individuals to solve their own problems." Sure, but the problem is the externalities. We don't say 'it's up to factories to solve their emission problems on their own' either - well, we do, but we impose limits on their emissions. It's politically (and arguably morally) infeasible to do the same for weight, so we do the next best thing.