Because it's a non-solution to a well-defined problem that just dumps the externality onto the consumer. Like telling people who are complaining about pollution that they shouldn't live near factories.
People complaining about cavities should stop eating so much sugar. Brushing their teeth is only a band-aid and doesn’t attack the core problem.
And yeah, that means they should probably stop eating processed food with added sugar. Which means they should avoid most of the shelves in the supermarket.
It’s a serious problem which goes far beyond personal responsibility. Our whole industrial food system is designed to deliver as much teeth-rotting food [as a bonus, also causes diabetes and heart disease and obesity] to as many consumers as cheaply as possible, while convincing those consumers that there’s nothing wrong. Not only that, we subsidize it heavily using our collective tax money.