It might be just because that's what they're used to getting from their peers. When a parent talks about how his toddler is learning to walk he gets a lot of "Wait until he gets big enough to start climbing on chairs and getting on the counter!" If you talk about your kid learning to ride a bike you get "Just wait until he's old enough to drive, then you'll REALLY start worrying!"
I think it's kind of a way to establish superiority of experience, something of a one-upmanship thing. And we single, childless people are easy targets. Everybody knows WE don't know anything about being busy or tired... we're single, we don't do anything with our time except watch TV anyway.