Why?
Simple, the diagnose is symptom based. If you recall your visit to meet the doctor, what is the first question they will ask? ( excluding greeting.... how are you... how is your lovely sister ..bla..bla..bla)
"What is your sickness/problem..." ask the doctor

Then, you start to explain how do your feel....... pain, dizzy , fever , cough...etc
After that, based on your explanation, the doctor will check either your temperature, BP, heart beat, lung .....
Next, the doctor will tell you the story " ........ it just a virus fever.... take this tablet ........ "
End of the story 1
Let say your sickness didn't get well, you'll either go and see the same doctor or you will see another one and start explaining again. The doctor might change the medication or ask you to go for some other test ( blood test, x -ray, CT scan....).
You may end up have to visit the specialist too. Now, here is the scary part. Once you are send to a specialist, the testing will be narrowed down to specific part of your body , either lung , heart, nerve system, ENT ( ear, nose, throat) etc. If your sickness is also contributed by other parts of the body, most likely it will not be detected.
One of my life experience is of my father. My father had a backache and numbness on the leg for quite some time. Following the normal procedure, the doctor prescribe some ointment to reduce the pain. After a week, the pain is still there. My father went to see the doctor again. Now, he prescribe pain killer and ask my father to go for an x- ray. From the x- ray, they found out that that one of the spine join cartilage has 'wear out'. This causes the two spine joint pressing on the nerve which cause the pain as well as numbness on the leg. So, the specialist says, my father need operation to put some metal plate to hold the two spine joint together. Of course the cost is high because the metal plate is made of titanium.
Well, after thinking for some time and since the pain is bearable, my father decided not to proceed with the operation.
Few month later, a friend of my father's friend (In Malay , we call it "Bapa punya kawan punya kawan") introduce to a MBBS doctor who also had learnt alternative medicine. He uses the pulse reading method. After diagnosing my father, he says that the numbness and the backache is not related. The backache is cause by muscle tiredness where the back muscle try to hold the position of the two spine joint. He taught some exercise to strengthen the back muscle. As for the numbness, he said it happen because lack of blood flow to the lower part of the body. He prescribe the herbal medicine and after taking the medicine for three month, the numbness gone. The back pain is much less now.
So, don;t trust your doctor 100% especially if it come to surgery. Get a second opinion from other doctor ( better if they are your relative or friend).
If you are interested in the pulse reading diagnosis, you can check the detail in my blog under the category of "Free Medical checkup" or you can click this link whic will direct you there