I think ppl do it because they think it's what you are suppose to ask!
Like just after you got engaged, ppl ask: when are you getting married
When you get married, ppl ask: So when are the babies coming
After first bub, ppl ask: When are the second one coming
Before you know it, ppl ask: So when are the grandkids coming!
I always ask how much a baby weighed at birth before I can stop myself, it slips out!
Just answer, and move on to another topic.
I also think it's a good way to learn patience. PPL asking you the same question over, and over, and over again Because believe me, that is the one thing you need with new bubba, not even thinking about their "why-phase"