Although that's not necessarily true either. Insurance contract typically pay for FDA-approved indications, but they also usually have some process where they will pay for off-label use if you can submit evidence that it's scientifically justified and cheaper alternatives would not work. Of course this is pretty stressfull, but it's really necessary in the U.S. because getting FDA to approve a new indication is really expensive for the drug companies, so there are a lot of drugs which are not approved for some of their main uses. For example, SSRIs are commonly used for anxiety even though this is off-label.[1] And monoclonal antibody-based immunosuppressives like adalimumab are often only approved for a few conditions, like Crohn's and arthritis, even though they are useful for auto-immune diseases in general.

In the specific case of ketamine for depression, some people do seem to have successfully gotten insurance companies to pay for it.[2] Even without FDA approval, this would probably get easier over time, as more studies show effectiveness.

[1] https://www.aafp.org/afp/2003/0801/p498.html [2] https://www.ketaminewellnesscenters.com/getting-insurance-to...

If insurance companies will pay for esketamine anyway, wouldn't they be more inclined then to also just pay for ketamine if it's cheaper?