Originally Posted by
raster
I think everyone replies with their job, whether or not asked specifically, because that's what people do with a huge portion of their time. I think, also, that some people don't do anything but their job, and "oh, I watch tv" doesn't really start a conversation.
And because colloquially, "what do you do?" usually asks for your occupation. It's a fair enough assumption that, unless it's otherwise made clear, this questioner expects job-talk for an answer. Sure, sometimes the savage literalists are looking for a list of biological processes, but that's a somewhat rare event.