I am certain most people have uttered this automatic response to the standard elevator/hallway question, "How are you?" I am equally certain nobody thinks about the real meaning of the phrase. It's just a common response to a common question. No effort required.
Does the asker really care about the askee's reply anyway?
The "I'm fine" response gives me pause because it is never true. For me, the honest response is probably "I'm in pain and fatigued, under a lot of stress trying to figure out my life." I am never "fine," but the phrase serves as both a mask and a conversation starter/ender, depending on the situation. I can hide behind these two words because people view them so carelessly.
I say it because it's easier than trying to explain RA, which can't be seen. If I were to say I'm not feeling well, the other person would look for clues to confirm this, and likely ask more personal health-related questions. If the other person can't see any indication of illness, he or she isn't going to take me seriously anyway. So I say it out of courtesy and go on with the day, fully aware of the odd and misleading phrase.