If we are hearing without thinking, it may be the case that we are not listening well. And you may know from personal experience, it isn't always easy to listen. Perhaps the lecturer is speaking too softly. Maybe the way the lecturer speaks makes it hard to understand what is being discussed. Lecturers sometimes speak too slowly or too quickly or end up talking on tangents that may or may not have anything to do with the theme of a lecture.
Some students even complain that the lecturer just reads out what is in the book.
As you will see, there are many potential obstacles to listening effectively, some to do with you as a listener and some to do with the professor as lecturer. If you want to benefit from the lecture, however, it is often up to you to take responsibility for improving the situation. One very powerful way is to learn to listen more effectively. As I mentioned briefly above, listening is a thinking activity and as such can be improved with instruction and practice. It is very helpful to define what activities you are actually mentally engaging in when you listen actively.