Maybe the US's federalism is a part of it too. We learned a lot of 'stock' law from hornbooks too, but there was always a caveat that most of the 1L stuff is state law and may-no, will-vary from jurisdiction to jurisdiction. The 'law' varies quite a bit from state to state with regards to criminal, contracts, property, etc. So you can either produce a great California lawyer who will be confused if she moves to Arizona, or a lawyer who understands how to learn the law and reason about it in general who can adapt to any state's idiosyncrasies.