Believe it or not you’re not alone!
My grandchildren have the same issue and no they are not junk eaters. The dentist explained that even the carbs such as bread that they eat can be the cause, as we know carbs convert into sugar. Another factor was the drinking water, so now they drink bottled water only. Brushing teeth more than once a day is also helpful.
Genetics are not ruled out but any protective measure must be tried.
Good Luck!