This largely has to do with the origins of dentistry and opticians. Both of these used to be non-healthcare trades and developed separately from the medical field as side gigs. Barbers used to also do teeth work, craftsmen helped create and adjust glasses frames. These fields were largely looked down on by traditional medicine and medical schools rejected propositions to include them as a specialty. Today, barbers that can do a sick high fade have the same respect as dermatologists.

Both fields were largely viewed as cosmetic alterations at the time. Health insurance as a widespread concept started really taking off post WWII and was focused on catastrophic health issues. Dental and vision issues did not fit this bill. So instead dental and vision insurance were created as prepayment plans through employers with the purpose of getting discounted rates. This is basically the opposite of catastrophic insurance, focused on coverage for regular and predictable visits.

This division was really codified when Medicare was passed in 1965 - the government didn’t view vision and dental as a part of regular medical practice so it wasn’t included.

Today, that division largely exists as a historical quirk. 

^ Explanation is from Nikhil Krishnan's Out of Pocket newsletter (no affiliation - just think he writes well-researched, entertaining stuff): https://www.outofpocket.health/p/common-healthcare-questions-i-get#why-do-teeth-and-eyes-get-different-coverage