Break a bone and chances are that the doctor setting it for a cast has seen something like it before. Contract some sort of bug, and it's just as likely the doctor knows what sort of treatment will get you over it.
The same cannot be said about a brain injury. Each one is unique. As the head of the Brain Injury Association of Missouri puts it, "Once you've seen one brain injury, you've seen one brain injury." Recovery is not a sure thing and if recovery occurs, it can take months or even years.