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The simple answer is that a young lawyer (barrister, in particular) learns much more by shadowing a senior lawyer in a legal chamber (legal firm). This hands-on or applied aspect of legal work is considered a privilege, certainly much more valuable than another year or two reading in law libraries (given that they will be doing it for the rest of their professional lives).
The technical term is βpupillageβ. In fact, in some parts of the world, being a pupil at a good chamber is still highly valued.
For more: https://www.barstandardsboard.org.uk/training-qualification/becoming-a-barrister.html