The idea behind insurance is that you pay a small amount consistently over time so that there is enough money in the mutual pot to cover it when rare expensive

The idea behind insurance is that you pay a small amount consistently over time so that there is enough money in the mutual pot to cover it when rare expensive events occur. Dental insurance is not included in standard insurance, and is in general a much worse deal, because all people tend to have dental issues in some form or another. So there is nothing rare about it to spread the risk / cost out.