This is more of a cultural question than a language question. "How are you" is a standard greeting in English. However, the person asking this question does not really expect any kind of a detailed response. It's just like saying "Hello" or "bonjour" (in French). If a stranger asks you "how are you" , you are expected to respond with a standard response such as, "fine, and how are you?"
If you were to actually tell the stranger what is happening with your health or the health status of somebody in your family, it would be taken as some kind of a joke.
During my youth, we would respond to this question by saying "fair to partly cloudy" as though we were giving a weather report. It was a sarcastic joke.
"How are you" is really an unusual thing to say to a stranger, if you think about it. However, that's just how we say "hello" in English.