Law professors vary greatly in experience, and I wouldn't try to make such a sweeping statement. Lots of top professions perform as adjunct professors. During my own time at school I had classes taught by a judge on the state supreme court, by a practicing patent attorney with decades of experience, a very experienced tax attorney, and by very experienced constitutional law professors. But there were also instructions who had spend nearly all of their time in academia and had limited experience practicing law.
One peculiarity about law schools is that the higher ranked law schools don't teach much about the actual practice of law, and tend to concentrate on teaching the principles behind the law. Those schools teach a generalization of the law and don't teach the black letter law even of the state. But most of their graduates won't practice in state anyway. By contrast, the lessor known, regional schools tend to focus more on being able to open a law firm in the state in which they reside one day after graduation.
But guess which set of graduates law firms want to pay huge salaries to upon graduation. Not the ready to practice graduates who know how to practice in state, but the graduates who can be apprentices in any state.
My experience as an engineer is that the typical engineering graduates ready to be an apprentice and that no one would expect a newly graduated civil engineer to be able to design a bridge or any other project impacting public safety on his own.
Under a government which imprisons any unjustly, the true place for a just man is also in prison. Thoreau: Civil Disobedience (1846)
The apathy of the people is enough to make every statue leap from its pedestal and hasten the resurrection of the dead. William Lloyd Garrison