A lot of REST APIs are just as hard to grok as GraphQL is as a whole. Companies often lack schemas and documentation, which GraphQL helps with out of the box.
It is not always obvious (e.g. strep throat is similar, but one or the other sicknesses may or may not need medicine) and doing things on a hunch is not how scientific fields should do things when there are simple tests.
Testing also provides safety for doctors against medical malpractice lawsuits.