They don't understand how medicine works. At best, it's an extremely educated guess based on your symptoms. This will give the doctor a general idea of what the problem is. The doctor will then rule out all the unlikely causes, again based on the symptoms. So for example, if a fatty comes in complaining of joint pain, the obvious answer will be to lose weight, as that is the most likely cause of the knee pain. If the fatty actually loses the weight and still has the knee pain (unlikely that it would lose weight, I know, but let's just pretend!) the doctor would then explore less likely causes for the pain. If a person with a normal BMI comes in complaining of joint pain, it's obviously not caused by too much weight bearing down on the joints, so the doctor will move on to the next likely cause. This is how medicine works. This is what fatties will never understand.