It's because some teacher somewhere along the way came up with this writing advice, which we all received in high school: "First, tell them what you're going t

It's because some teacher somewhere along the way came up with this writing advice, which we all received in high school: "First, tell them what you're going to tell them. Then, tell them. Then, tell them what you told them." Following that structure leads to terrible, boring writing. The only useful part is the second sentence.