Docs are too busy to get paperwork done. They save lives, and it wont be fair to ask them to document everything themselves. They could be given an assistant, but when they can get it done cheaper with a transcriptionist, they would naturally opt for it given the volumes of reports and records they generate every day.
The docs have to dictate stuff anyways as there is no way that the stuff inside their heads are going to magically come out other than through their mouths. So the bullshit that comes out of their mouth gets recorded into their computers, and the recorded files are transfered back to companies that are popular among Medical Transcriptionist Jobs seekers, and they convert it back to record and email it back to the docs.