I don't think it's a hard and fast rule that you have to answer "I'm fine", is it? What's appropriate as an answer will depend on the person asking and the context they're asking in.
If you're meeting a friend you've not seen in a while, then "How are you" is likely to prompt quite a lot of detail about what they've been doing recently and how they're feeling, and further conversation develops naturally from there.
If a colleague who you don't know well personally asks you, then you probably want to keep it much briefer, especially if you're both quite busy just then. Still, there's no reason to have to say you're fine. Some answers- "I'm currently experiencing relentless existential dread", and the like- aren't going to be appropriate, obviously, but there's still probably a fair amount of scope in answering.
The only times I'd reflexively go "I'm fine" are if I really don't want to make conversation or if it's people I've just met and have absolutely no connection to. Even then, if I feel like shit, then I might say something along the lines of "could be better".
There's no one right answer. I know plenty of people who, if asked, will launch into great detail about how they're doing, even if they barely know me. And, unless I'm in a hurry or something, I don't usually mind. I'm obviously not as invested in their lives as they are, but I usually find it interesting to get to know more about people (most of whom don't seem to care that I'm "making a mockery of empathy").