There are a few reasons why TMJ may not be covered by insurance. One reason is that it is considered a pre-existing condition. This means that if you have had any symptoms of TMJ before you got your insurance, the insurance company may not cover it. Another reason is that TMJ is considered to be a cosmetic condition. This means that the insurance company may not feel like it is necessary to cover it. Finally, some insurance companies may simply not offer coverage for TMJ.