The thing is, though, grumppa, that the phrase how are you isn't really asking after a person's health, is it? It's really just a polite way of saying hello. Nobody wants a long spiel about a person's actual state of health upon greeting. And so we reply, "Fine, thanks", even when we are not quite fine, because the idea is to get over the hello(e)s and on to proper conversation or to move one swiftly 
Fine, thanks
I'm good
Fair to middlin'
etc
are equivalents. In my view, people who complain about changes to such trivia are just moaning and showing their lack of tolerance and adaptability (because of age, perhaps?).