[marssaxman]

I realize that it can be interpreted as rude, I just don't understand why. It's not a question about socioeconomic status, it's a question about "what [you] like to do, what interests [you] have, etc.," as those characteristics are expressed in the activity you've chosen to spend at least half your waking hours on.

[mac01021]

A large fraction of people in the "developed" world (perhaps a majority?) work as something like a cashier, for an employer they don't really care about. And they are all aware that most other people they meet are working similar jobs. So they don't ask, because they know they'll probably get a reply like "I stock shelves in a department store", which isn't going to lead anywhere interesting.

Because of this, if someone does ask them what they do, they can be pretty sure right away that that person does something "better" than what they do, and (wrongly or rightly) it can make the situation uncomfortable for them.

Obviously the extent to which this is relevant depends on the demographic of the group in which you're socializing at a particular moment.

[eropple]

You're replying to someone who explained to you that that question is, in many quarters, a question about socioeconomic status, and your reply is "I don't understand why it's rude, it's not a question about socioeconomic status." Yes, it is about socioeconomic status. You may not think it should be, you may not like that it is, but it is, otherwise people would not take it as such. The reasons are historical and cultural, and if you study the (American, though not exclusively) relationship between work and sociological sense of self of the 20th century you are surely more than capable of divining the details, but it exists, and your further confusion about rudeness will probably continue until you accept that the measuring tools of rudeness are not yours to calibrate.