Now
why don't us humans have the 6 layers and the constantly replacing teeth.
Some dentistry work on a 70-million-year-old tooth has provided an insight into the evolutionary success of duck-billed dinosaurs. Hadrosaurs' unique tooth structure is now a vital clue in the mystery of how the billed herbivores, dubbed "the cows of the Cretaceous era", spread so far and lived for so long. The ancient …
The fossil teeth are not the duckbill's teeth, they are fossil teeth: they are made of rock and replaced the original material in a way that we don't completely understand yet. How can they tell the wear rate by scratching on rocky material that replaced the original material?
"they are fossil teeth: they are made of rock"
As opposed to "living" teeth which are made of what... meat? Hydroxyapatite is a naturally occurring rock, too!
All the organic components of the tooth will likely be long gone, but much of the mineral content may still be present after fossilisation. Given how long the hadrosaurids were in existence and how widespread they were, there will no doubt be many, many fossils to choose in order to find one with a plausible mineral content or even ones which still contain proteins.