For dental, at least, it’s because there are two ways to treat issues with your teeth: extract them or repair them.

If you go to the emergency room with tooth pain because of some cavity that gets infected, a doctor there can and will extract it. And your insurance will cover this as a medical expense (unless the doctor was an actual dentist and charges as such). That’s also why wisdom tooth extraction is often covered by medical insurance.

But if you want to preserve that tooth, you need a dentist, with specialized skills and tools, which are far more expensive.

Insurance companies get away with not paying for dental work because “technically” you don’t need your teeth to eat, and “technically” you don’t need all of them to chew, and “technically” you can be perfectly healthy without any teeth at all. QED, they argue trying to save your teeth is a cosmetic expense.

And they got away with that reasoning. And they still do.