You hear it every day, yes? What the heck is this repetitive script really about? Ah, glad you asked.
It's a tribal affiliation assertion: I'm extending you temporary membership in my tribe. Probably only for this encounter, but who knows.
I'm saying "I see you". Most of us live in a world of way too many people, and that means we have to treat them like trees or rocks because they can't all be human beings. So -- I see you. You're not just a rock.
And it's an initiation protocol. It acknowledges that we're not just walking by but beginning some kind of exchange or transaction between us. We're engaging.
When taken literally the question has complicated ramifications. But it's not a literal question and few people want detailed (or long) answers.
"I didn't sleep at all well and I'm feeling some vague sense of underlying dread or maybe it's existential ennui or maybe indigestion and really is this all there is and I wonder whether I'm getting enough fiber. Thanks for asking."
No, I don't recommend you answer in detail because it breaks the (lightweight) social protocol and, frankly, I've done it, and I can assure you that it doesn't make you popular. Also, it defeats the multiple, socially essential purposes of this ritual.
But yes, also, I am indeed suggestion subversion. Subvert the script in your own head. Listen to yourself say the words. Notice what's really going on.