It's a difference in mentality between old world Europe and America. Even in the 21st century some aspects of Paris (where I live) are still classist. And the most straightforward way to determine class is by your occupation.

Americans ask what do you do because we are, honestly, interested in what you do all day. I work in computers, after work I enjoy discussing computers, tech, drinking. In a previous life I was a bartender, wind tunnel tech, and traveler. But I don't do that now and if I wasn't behind the bar most people wouldn't know I lived that life. So what do you do is an icebreaker for me.