The first reason is that no two experiences are exactly alike. No matter what I have to say about the way that my labor and delivery experiences went, yours will go very differently. Discussing my personal experiences will do nothing to prepare another mommy for what is to come. In fact, hearing the horrific details of my labor/delivery will only scare those who read about them. This alone is a reason not to share.
The second reason to skip this part of the progression is that, if you are pregnant or even know someone who has recently given birth, you have already heard all the details. This is because mommies are proud of what they go through when bringing their children into the world, and they love to share their personal experience with anyone who will listen. I know this because I am one of them. I will share everything that I can remember about the times when my sons were born, with anyone who will listen, and I have noticed that the mommies I share my time with are just as open about what they went through.
As mommies, we are proud of how much we are willing to go through for our children, and proud of our sacrifices and scars. We wear these experiences like medals and are happy to re-live every part of it as a reminder of how amazing we are and how much we love our babies. If you are truly curious about the things that no one talks about, find a mom and ask. You should plan for a long conversation, so make sure you have a great deal of time to spare.