I ask it myself all the time. And I get asked this all the time. Of course, it's just a conversational convention. No one really wants the true answer, & no one expects one.
Right now the true answer is "I'm not well at all." I got the numbers from my complete blood count test (CBC). It was worse than I thought. About 10 days ago my hemoglobin was 7.9. When my health care friends hear that, they blanch. Jeeze1 That's low! Very low!