I mean you can buy it here in a normal store. So I’m not sure if you mean a dentist can’t use/provide it, or if you’re thinking about the nano forms of it.

Edit, just saw this:

the FDA regulates dental products like toothpaste as cosmetics rather than therapeutic agents for cavity prevention

Interestingly, many ingredients used in dental care, including fluoride, are employed off-label. This means they are used in ways not officially approved by the FDA but are still considered effective based on scientific evidence and clinical practice

For other toothpaste that still strengthens enamel, there is toothpaste with hydroxyapatite (which can be ingested, at least that specific ingredient). Though it is probably more expensive.

It could likely be replaced with hydroxyapatite instead (it also can be used to remove lead and other things from water, which makes searching about being added to municipal water difficult). Good for not only teeth, also bones.

I also wonder if adding other vitamins would make more sense (just enough to stop deficiencies) if we’re talking about health outcomes, though the first idea I had with vitamin C came up with results of that messing with the chlorine in the water.