What is the necessary entity that cannot be done by professor without physical classroom with assembled students?
Have you ever taught? When you explain something to a class, have you ever seen understanding happen in students' faces, or confusion? When the former happens, have you stopped your speaking and prompted the student to continue your thought and make it their own? Or in the latter case, have you stopped and asked what was unclear, had a discussion about it, and then tried to explain it in a different way?

None of this happens in an online setting: You give students a textbook chapter to read or a video to watch, but it's not interactive: You can't catch students where they need to be caught to learn efficiently because you don't have the personal interaction and contact. Personal contact really is a very powerful thing.