Comment by hello_computer

Taking compounds of the most electronegative element, and treating people with them systemically—drink it, bathe in it, cook with it—when local administration directly to the teeth is cheap, easy, & effective makes no sense at all.

Meanwhile, mining industries produce—and must dispose of—a steady stream of fluoride compounds that would otherwise have to be dealt with as toxic waste (i.e. very expensive) if they ever lost the green light to dump them into the water supply.

https://en.wikipedia.org/wiki/Harold_Hodge

or per Google:

Three chemicals are used to fluoridate drinking water in the U.S: sodium fluoride (NaF); sodium fluorosilicate (Na2SiF6); and fluorosilicic acid (H2SiF6). Fluorosilicic acid is a byproduct of the manufacture of phosphate fertilizer.