A substantial fraction of the variation in dental caries is independent of behavioral risk factors.
2 kids, same diet, same hygiene regime; one flawless set of teeth, one mouth full of fillings. (In fact, the bad teeth get even more care than the good teeth).
Some people just have weak teeth.
Yes, that's what I was saying.
Dental insurance is a bad deal. It mostly involves paying in advance for dental work, at a premium. Note that routine dental work is not something that "insures" well; you can't buy cost-effective insurance for something everyone knows is going to happen.
Plus you make a good point on why people should avoid junk food and exercise better hygiene, and teach they're children to do so as well (I know that's not always the case, but I bet studies show it is a huge majority of the time).