I'm sure that this is caused by a number of reasons - some native american groups have very thick enamel, some groups have saliva that is naturally antibacterial for the types of bacteria that cause cavities (but from what I understand, this second trait is also associated with heart disease.)

It might be better to investigate these things based on the assumption that "G-d knows where" isn't a single place, and that all people that one considers primitive aren't the same people.

For example, you ever wonder when they show primitive people and their teeth are terrible, or when people that live in your culture spend no time on their teeth and never visit the dentist, but their teeth are fine?

Or how some people are naturally thin, and some have bone structures that seem like they were made to be fat?

Some form of dental hygiene is common worldwide. So, you can find young people anywhere with a clean smile. But, the worst cases get really bad.

Sadly, you will often see 40 year olds missing many teeth in those same areas.

I think a big part of it is that once you have even a minor cavity, eating anything sweet hurts like hell, so people just don't eat sweet stuff when they're older and have lost their baby teeth.

The feeling of a candy bar getting just right into the cracks of your molars is a great motivator for avoiding such things.

No refined sugar in the diet certainly plays a big part.

This interesting video talks about that. Time stamped to 6:51

https://youtu.be/zbzT00Cyq-g?t=411

No refined carbs → No cavities

Tough food, lot’s of chewing → No crooked teeth

Natural selection. If you got fucked up teeth out in the wild you are screwed. If you got fucked up teeth in civilization since the 1700s theyd just give you pig teeth or wooden teeth or whatever to get you chompin again.