It's not all of medical science, no. We know a great deal. But that only pertains to general study, and not specific cases. We don't know shit about someone, medically, until we do tests. Even then, we can't do an in-depth dissection of them (because that would be wildly inhumane) so we can mostly only go off surface level information. Even for more in-depth information, say with x-rays and MRIs or blood tests, it still only general knowledge. Each person is unique, and has unique characteristics. So we need to take what information we have and try and match it to previous cases to determine what it could be.

Sometimes it's really easy. "You have a cold, go drink some water and get some rest". Sometimes it's not, they have some obscure neurological disorder that only affects .0000001% of the population (at a guess).