Although I enjoyed reading this, I would offer a much simpler explanation as to why people ask this question: because it's obvious. People are generally not very insightful, yet when in a situation where questions are to be asked, there are always some people who feel the need to hear their own voice so that others recognize that they're in the room. It's something I see a lot, and is independent of the subject matter at hand. The place these people will start from hinges on how self serving they are. Mild narcissists will ask the obvious question. Hardened narcissists will ask a question that makes it obvious how smart they are, no matter how little bearing it has to the discussion at hand. Asking a childless woman why she lacks children isn't an indictment of society's views on women (although it could be; hence the interest in reading the subtext), but rather a feeble attempt at being heard.
The same exists in daily life. I've been married for a year now, and if I had a nickel for every time someone asked my wife and I when were having that first kid, I'd be rich (or at least I'd drink espresso all day long and forget about this cheap drip coffee bullshit that I'm currently drinking). It's enough to give my wife anxiety from time to time when she's around my family, as they can be quite aggressive in their inquiries. But they're not really being rude to their eyes. They're just making small talk.