I'm pretty sure prehistoric people did have primitive tooth care, didn't eat junk food and high-sugar diets, and still had nasty teeth.

Genes, climate, and types of local foods play a role too, but widespread adoption of agriculture made diets rich in carbs and starchy foods possible, which isn't great for dental health, and the somewhat recent trend (last 200~ years) of highly processed foods that are softer and easier to chew means there's less mechanisms to deal with tooth overcrowding. This has been observed in children from hunter gatherer societies, raised on modern diets, having worse dental hygiene than their parents.

They had two sets of teeth and they were perfect for the average life expectancy of around 35.

> Teeth-cleaning twigs have long been used throughout human history.[2] As long ago as 3000 B.C., the ancient Egyptians constructed crude toothbrushes from twigs and leaves to clean their teeth. Similarly, other cultures such as the Greeks, Romans, Arabs and Indians also cleaned their teeth with twigs. Some would fray one end of the twig so that it could penetrate between the teeth more effectively.

https://en.wikipedia.org/wiki/Tooth_brushing