Why does heroin still exist legally?

My grandfather just passed away from liver disease and is getting pain medicine, and a good chunk of it is codeine and heroin laced medicine. He’s taken it before and the docs know he’s addicted to it and they still prescribe it to him and give it to him. Why would doctors give him medicine that they know he is addicted to? And I just don’t understand why heroin is so commonly used, I get that it is medicine, but we have other alternatives now and it’s such a dangerous drug and just kills so many people. I can’t see why it is still legal or used at all, but I am just a dumb teen. Any information is appreciated.