Dental and vision insurance are tricky, price-wise, because the costs are usually either low OR predictable and delayable. Regular dental and vision exams are cheap. Glasses are very predictable and can be cheap or expensive. Stuff like fillings and root canals is also very cheap compared to, say, knee surgery.
But then there are properly expensive dental things like implants and dentures, and generally someone knows ahead of time that they're going to need dentures or implants, so the sensible thing to do, as a consumer, is to *not* have dental insurance until you need something major. This is a bad deal for the insurance companies, which is why individual dental plans are a bad deal for consumers.
Only people who wear glasses are ever going to pay for vision insurance. So everyone is just sharing the cost of each others' exams. Which all cost about the same anyway. So why have insurance?
FWIW my health plan (through my employer) does include vision and pediatric dental care as part of the main plan (we have a separate dental plan for adults). The pediatric dental part is mandated at the state level (by the Commonwealth of Massachusetts); I'm not sure if the vision part is too.