It might be also potentially dangerous for bacteria build up because of space in between layers. 3D printed stuff is certainly not safe for food.

Resin 3D MSLA printing. It's how they make invisaligns, the shape is one piece of plastic. You can buy dental application resins.

https://dental.formlabs.com/

Dental 3D printing is a different beast from consumer printers. The dental printers are already certified to actually create regulator approved devices that can go in a patient's mouth.

I imagine it’d make more sense to print a mould and cast the teeth.

they probably also need to account for patients differences in individuals mouth, teeth, jaws that correctly fit.