My best friend got robbed at gunpoint in high school. When our teenage classmates found out about this, they couldn’t believe he surrendered.
“You should have knocked the gun out of his hands!”
Sure. Why not? Despite his zero hours of self-defense training, Jacob should have roundhouse kicked the pistol away and tackled the guy.
Writing a book is a similar situation, albeit with much lower stakes. “I could write a book!” people say. Then, the blank page points a gun at them and they panic. They may write a few sentences and then stall.
It’s very difficult to do what you don’t know how to do, especially when the pressure is on.