Maybe because every child is different!
I never thought I'd have to sit down and have the talk with my kids and it was something I never really wanted to do. I didn't with my son -- he figured it out on his own and was fine with it. My DD wanted my confirmation and she asked me flat out, begging me to be honest and serious with her, so I was. She'd asked in the past, but she'd never been so sincere before. I wasn't about to sit there and prevaricate when she was so dead serious.
That, IMO, would have been completely wrong.