One big difference is that a classroom has 30 or 50 or 100 students, and an in apprenticeship environment you can have probably up to 5 students. The classroom is much cheaper.
Another difference is that in an apprenticeship you can "fire" the students. In a classroom there is a big pressure to keep the students there.
Also there is a different psychological dynamic in small groups... The teacher naturally knows and cares more about the student, and the student feels socially accountable (ie. "I want to make (so-and-so) proud."
And, in a real sense, the master and apprentices are a team.
And this isn't the "we need more money and teachers" argument I'm making, just that the large-group lecture format only works well for certain stages of learning and types of learners.
Another difference is that in an apprenticeship you can "fire" the students. In a classroom there is a big pressure to keep the students there.