Most chronic illnesses are the result of accumulated damage and dysfunction that are diffuse throughout the body. Something like MS has done damage to millions of nerves by the time it gets diagnosed, and the body is not particularly good at healing nerve damage to begin with.
Chronic illnesses almost always require chronic treatment because of the nature and extent of the damage.
ah it seems i’ve misread the initial post and replied to this under the assumption it said “rare illnesses have no cures”
Quite the contrary, there are some rare illnesses that are unique in that they do have actual cures now. For example, the Sickle Cell gene therapy is the first gene-editing treatment to be approved for human use and completely cures the individual patient of Sickle Cell disease.