Fluoridated water doesn't seem to make a difference on cavities. It does have neurological effects. It's simply not acutely fatal. It's already in our toothpaste. We don't need it in our municipal water supply and the majority of developed countries don't.
https://www.hsph.harvard.edu/magazine/magazine_article/fluoridated-drinking-water/
Fluoridated toothpaste is more effective than drinking water. The fluoride works by direct contact with the enamel. Another reason it doesn't make sense to put it in drinking water.