How are you?

As Wikipedia says 'politeness' is cultural thing. What is considered polite in one country can be rude in another.
My cultural background is the root to my confusion.
When people ask "How are you?" how come they expect to hear something like "good," "fine," "ok" or any other form of saying that things are ok? What if they are not?
Like today, a friend of mine is not really happy, he's miserable (as he said), but whenever someone calls he says "I'm good."
So how close you should you be to a person to stop lying about things?
Or this question does not have any meaning and used not to learn something, but just to "play" conversation according rules of the society? I guess that's how it is.

Still... if people do not really care why would they ask? Wouldn't that be more polite not to ask at all if you don't care? I guess that would be culturally unacceptable.