Then why ask it??? To fake empathy, perhaps? If a person doesn't want to hear my answer to a question, I suggest they just don't ask the question. Saves everyone a lot of time, and avoids plenty of confusion.
Otherwise I'm just going to continue answering questions in a logical, honest and forthcoming manner, perhaps appearing (to some) as lacking "functioning social skills".
The only people who ask me that question in that specific way are strangers, mostly during solicitation calls or any other calls where they have to read from a script.
As per your suggestion, next time I get a marketing call, I might just start delving into the details of my life for 10 minutes, explaining exactly just "how I am" :P
As to why it exists, the answer is the same for just about every other linguistic feature: nobody really knows.
Positive on the Z. Fight me.
In all seriousness though, nothing drives me more nuts than asking questions you don't actually want the answer to, or are just asking to fill the silence. It's gotten to the point I ask people if they are really interested in the answer, or are they just looking for small talk,
Generally, I'm happy to talk with people, but I am not a big "small talker", because I'm generally a "communicate to problem solve, otherwise do" type of person. Building a relationship with someone requires over time I consciously build and try to track conversational contexts to facilitate faster, more efficient communication.
The process of doing that requires a significant investment of time not just to speaking, but to listening, building and working through conjectures, and familiarizing myself with how they approach problems, and determining how differently they do so from me
All this information rides along in my head all the time. The friendships and working relationships you build through such a process are unmatched. However, it takes significant effort, and it can lead to some awkwardness when you start actually noticing where the rougher edges are on who you're talking to.
I'm not one to leave a rough edge unsmoothed, but committing to help someone smooth out a part of their character in a way you can actually productively help is also a non-trivial task.
Some level of all of what I've described goes on in every encounter.
This makes people who ask questions they're not really looking for a true and substantive answer to incredibly annoying. It paints the picture of a person using me as a means to soothe their own discomfort with doing whatever they need to do. It's...trying on the patience.
Speech is not merely a rational exchange of information. If you try to understand every speech act in terms of the information content being exchanged through the surface meaning of the words, you're going to be confused by a lot more than just greetings.
When someone says "How are you?", I simply have it difficult to just say "very well thank you", if I'm just anticipating a root canal treatment, or whatever.
We Finns can indeed appear a bit autistic; it's not meant to be rude, but may feel like it.
If one is in France and says "Ça va, Michel?" -- odds are one expects a little more diversity in the answer than "How are you?" expects in the US... but really, one still expects "Oui, ça va." or "Ça va bien" and not really "Ça va mal" or "Ça va pas."