IIRC it’s because there isn’t really much of a point to add those to insurance. With health insurance some people will need very expensive treatments but lots of people don’t. It works because you spread the risk over many people. The people who don’t need expensive treatments pay more than they would without insurance, the ones that do need those treatments pay a lot less. Since you don’t know which one of those you will be insurance is a good idea.
With dental and glasses this is not the case. There isn’t too much variation in how much a person will need to spend on those during their lifetime.
If you get additional insurance for either you’ll see that the maximum payouts are pretty much the same as what you pay extra during the same period. You might as well just put the money in a savings account.