Basically imagine you are perfectly healthy, and you take a drug that has a 99.9% chance of increasing your IQ by 20 points, or a 0.1% chance of reducing your IQ by 5 points.
The FDA would basically say "no way in fucking hell, you don't need the extra IQ points to cure any problem, and there is a risk of brain damage".
Now if that same drug was instead marketed towards helping people with traumatic brain injuries heal, then it could maybe get approval.
There was a petition a few years back to the FDA to change this to allow research into life extending drugs, but AFAIK it didn't go anywhere.