Some doctorates are more specific than a PhD

Some tasks require specific qualifications for which a generic PhD may not suffice. For example a researcher working on neuroscience or robotic prosthesis may come from various fields of science, but (depending on your jurisdiction) can be prohibited to work on people independently before obtaining also a degree in medicine - no matter what skills they may already have, the specific degree is mandatory. Similarly, there are areas of sociology and politology where a reseacher would be well skilled in the relevant areas of law, but they may need to obtain a jurisprudence degree to be allowed to practice those skills.