Is it because of the years spent mastering the terminology? Is it because of a blind ignorance to the fact that not all clients are "legally trained"? 

Nobody can be sure of the exact reason; however, what everybody can be sure of is the need for the profession, as a whole, to use plain language as good practice in all business documents. Not only will it help with productivity and efficiency, it will also help to reduce the risk of misinterpretation of advice given.

As Marcus Fabius Quintilianus points out:

“Don’t write merely to be understood. Write so that you cannot possibly be misunderstood.”