I don't think eczema is only a symptom it's not like a migraine although, it can be the case. 
I wouldn't call the result of a spider bite or skin allergic reaction eczema although, it officially is. Eczema is dermatitis which means it's an infection and some are very serious. Same if it comes to itchy and itchy... so itchy that you can see the bone is a serious problem. 

The question is indeed why eczema isn't worth analyzing. Poop, pee, blood, and even hair is a source of information, so are the life circumstances and the fact chronic diseases change DNA.