The central tenet of insurance is to pay slightly more on average while getting protected from paying very large amounts of money should disaster strike. In essence it's paying a premium to lower risk. A large company can pool risk, so it doesn't feel the effects of a small number of i.e. houses being burned down by a wildfire in California.
From this link, it's very common for dental insurance to have quite low annual maximum benefits in the $1000 range, in addition to a lifetime maximum. But major dental procedures with surgery can cost much more. The companies are very large risk pools, so they only care about average cost like most insurance companies. This means the only reason they impose benefit limits is because not doing so would significantly add to the average cost. Thus, a substantial fraction of dollars spent are spent beyond this limit.
This leaves two questions:
- Why are companies passing on so much risk to individuals, when they don't do so for other types of insurance such as houses that can cost upwards of a million?
- Why pay for insurance in first place if it can't protect you from the risk of very expensive procedures?