The human body can't turn dietary fluoride into harder enamel, it has to stay on the teeth, topically, for a while to soak in. As such drinking water is a suboptimal way of going about applying it to teeth. Fluoride in toothpaste is highly effective. Dentists applying highly-concentrated fluoride stuff directly to your teeth even more. In people who actually get their teeth made resilient by such measures fluoridated drinking water has exactly zero impact as the teeth can't get more resilient, in people who don't, well, it's something, a little step. There's a reason Europe isn't fluoridating drinking water: We don't have huge segments of the population falling through the gaps of the health system.

is cheap AF, and is completely trivial to distribute.

And if you were Brasil or India that would make sense. The US, OTOH, does not have an excuse when it comes to stingy with more effective measures: You have the resources to do better.