A teacher tells you what is important, where to focus, how to practice what you've learned, what to do next in order to become more knowledgeable. Textbooks are good, but they are usually not good enough, in the sense that reading a book, without further feedback, is not desirable to most students. Most students need summaries of what is important, and they need guidance on how to apply the important things. There is usually a desire for more interactivity than books can provide. Classroom instruction can be interactive, but it often isn't.
Students become passive observers of a performance by the teacher.
Teachers have to work very hard indeed to bring students into a dialog. It is certainly possible to create interactive mechanisms. We can animate figures, ask questions, provide answers, and suggest what students should do next. Without set meeting times, there are no classes to be missed, either accidentally or because of time conflicts with other activities. Missed lessons can be made up easily.