Its a pain in the arse to remove if you don't want it (RO or expensive filters that don't completely get it all out) and it's in mouthwash toothpaste already if you do want it. It also has effects for people using municipal water for other things where they also don't want it for example perhaps for animals and brewing.

Also if you read Weston A Price "Nutrition and Physical Degeneration: A Comparison of Primitive and Modern Diets and Their Effects" it becomes pretty clear modern dietary habits are the cause of modern dental woes. Mass fluoridation might help the teeth in some cases but doesn't fix the key problem and causes issues elsewhere so the idea of applying it to everyone's water is just bonkers..