i don't think this is something that can be answered clearly. i need to go on a hunch. so, don't take what i'm about to say totally seriously - yet.
it seems like the only way we know how to put these molecules - calcium, phosophate & a hydroxyl goup - together is using a specialized kind of cell called an ameloblast that disappears in early childhood. it seems like we can give our bodies as much calcium and as much phosphate as we want, but we don't really know what to do with it. we could maybe fix that with stem cells, by growing teeth in a lab and implanting them or with crispr technology - and that might not be very far in the future. but, for right now, this is a puzzle that eludes our biology. we see the pieces of the puzzle, but we can't put them together properly.
hydroxyapatite is insoluble in water, and thankfully so - otherwise our teeth would fall apart. but, solubility increases as ph decreases, because it reacts with the free hydrogen in the water. there seems to be a perception that this is reversible if you increase the ph in your mouth, but i think the point is that it isn't - that once we strip the calcium, and the phosphates, out of the lattice, we don't know how to rebuild the lattice.
but, we've figured out that the lattice seems to spontaneously rebuild in the presence of fluoride, and that we seem to uptake it quite readily once it does. for that reason, our teeth end up replaced by a different molecule - fluoropatite - that contains calcium, phosphate & fluoride. this substitution is actually beneficial, but others - such as replacing calcium with carbonate - are not.
but, we have little control over it. we don't know how to put the pieces back together, and will absorb screwy lattices when we put them back together wrong - both at our benefit and at our expense.
it follows, then, that we ought to be able to absorb a prebuilt molecule of actual hydroxyapatite, if we can find it - we can absorb it with fluoride (an improvement), and we can absorb it with carbonate (a worsening), so we should be able to absorb it the right way, too. right?
and, why don't we just fucking do it, then?
because the only natural source is animal bones, and we don't eat those anymore, or pick our teeth with them anymore, at least. some of us eat soup, perhaps. but, the molecule isn't soluble - you need to smash them up. you can't eat it either, exactly - it's topical. but, if you chomp on them long enough, you could release it.
your dog loves chewing on bones.
maybe it's on to something...