Studies have shown significant improvements in dental health from adding flouride to water. As an example getting kids to brush their teeth regularly is difficult, but we can provide a layer of protection and avoid expensive dental costs whenever they drink water.

Because that puts the burden on the individual.
For you that may be bearable. But for poor people, who have to choose between eating and brushing their teeth, that may not be as straightforward. Similarly small children are at higher risk, because they don't usually choose their toothpaste. So if their parents incorrectly believe that fluoridated toothpaste is harmful or cannot afford it, they might end up with tooth decay through no fault of their own.

Not everyone has great dental habits.