We're mass-producing teachers, but we're also mass-producing students. In the example of a medieval university or a traditional apprenticeship, the students have some kind of vested interest in being there. These were limited opportunities, not universal requirements. In contrast, most teachers in most classrooms today are trying to impart knowledge that their students aren't particularly motivated to have.

Lectures are a great way to convey information to students who want the information in the first place. They are a terrible way to engage with involuntary participants.