I think this is partly because dental insurance bears little resemblance to comprehensive medical insurance. In fact, for most people with a generally healthy dental history, it's often not worth paying for dental insurance, since it ends up simply being a pre-payment for services. Expensive procedures & oral surgery are often covered for only 50% of the final cost.

In the US at least, I think dental "insurance" is really just a simplified way for employers to offer an appealing benefit to employees.