You're making a perfectly good point: no one can figure out what will make people happy just by sitting in their armchair. But there are a lot of things we might mean by the word "happy" and if we just ask the person on the street if they're happy, we may not know what to make of the answer.
There's a recent short essay by Gary Gutting in the New York Times' The Stone series that deals with some of the issues here. and for present purposes I don't have a lot to add. But at the least, we'd want to make a distinction between the passing state of our moods and the condition of our lives overall. Being annoyed of an afternoon doesn't mean that I'm not happy, full stop. And being in a good mood on another afternoon also doesn't mean I'm happy, full stop. To which we can add: part of what Aristotle and other philosophers want to know is what sorts of things make for a life worth living; the word "happiness" is at best a rough translation for "eudaimonia."
There's another problem with just asking people if they're happy and if so why. As mountains of psychological research have made clear, we're often not nearly as good as we think we are at figuring out what's going on in our own minds. And even if I'm right in reporting that I'm happy in some sense or other, I might be wrong about why.
So yes: there's a lot to be learned about happiness by getting out of the armchair. But if we're going to look to the world, we need to have some well-thought-out ideas on what we're really asking and what would count as an answer. That part of the job isn't just for philosophers, but it's the part that philosophers are likely to have the most to say about.