Because it would increase the number of medications that interact with each other. E.g. you're taking medications A and B, but now A and B come with the potentiating compounds in the pills. So taking both of them could be dangerous. You to go to the hospital and they need to give you C, but it would be dangerous because of the potentiator. It would also make medication management more difficult as people have to remember if they're taking the potentiated drug or not.
