Primarily because it's the way most of us were taught. However, these days students are accustomed to a variety of lecture formats and styles. Changing the format of lectures from lesson to lesson is one trick to spice up the lessons. There are a number of ways to lecture students. Remember to keep lectures interesting and interactive or you may quickly find looks of boredom or puzzlement on students faces.
Ideas Use the Socratic method .
Ask the class questions and respond to students answers with yet another question. Work questions or challenge problems into the lecture. Elicit responses from the class on what they already know. Also find out whay they want to know, and design the lecture to provide them with that knowlege. Provide a partially filled in outline and then give a structured lecture to complete it.