Because people keep demanding that it gets cheaper so that tape gets sent off to India where it's transcribed by someone that barely speaks english.
Not true. Medical transcriptionists need to have a vast understanding of medical terminology, have to turn around transcriptions usually same day, and their transcriptions are reviewed by the doctors. Any outsourced contract that didn't provide above 90% accuracy would be cancelled by the hospital.
My sister in law and her mother are both medical transcriptionists for a company based out of Toronto (Canada). The pay isn't great, but the hours are somewhat flexible and they work from home. They're able to work from home because the recordings are all digital these days. The recordings go on to the server, tagged with the doctor's name and a session id of some sort. The transcriptionists pick it up from the queue, and type it in as they listen to it. They have an SLA with a minimum turn around time for transcriptions, based on the type of recording and duration (eg: transcriptions from a podiatrist might be fairly short and straightforward, while those from doctors in emerge or ICU might vary wildly in length and complexity.)
FYI - it's not easy work at all. Many doctors have different accents or don't dictate clearly, use different terminology or abbreviations, etc, and there are often many names for each drug (eg: acetaminophen is the generic drug, also called paracetamol, which might have 200 trade name variants such as Tylenol.) Hospitals have to pay licensing fees to companies that maintain drug information databases for them, such as Vigilance. Transcriptionists often run over the same recording multiple times to figure out exactly what was said, and have an open voice chat to each other to discuss anything they're unsure of. As a last resort they can send it back to the doctor to re-dictate, but you can see how that would be frowned upon.