The process for approving a new drug essentially requires proving that they are safe and also have sufficient efficacy, and usually also that the balance of the

The process for approving a new drug essentially requires proving that they are safe and also have sufficient efficacy, and usually also that the balance of the two (aka benefit:risk) is an improvement over a reasonable existing comparator. There are plenty of drugs I can think of (even relatively modern, specialised drugs) for which we kinda understand some parts of the mechanism of action, but not other parts. Also, given how difficult and competitive developing new drugs is, the incentive is to do the minimum (as above) for approval in the shortest time possible.