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.”
The legal profession tends to believe that it is better than average at communicating with its readers – clients, staff, prospects, judges and social media users. Regrettably, in many cases that confidence is misplaced. In fact Bryan Garner, a leading US advocate of plain language for lawyers, says that the legal profession suffers significant barriers to communicating including: "We persist in making our profession exclusive, all the while rationalizing our inability to write well by invoking age-old legal standards We have a history of wretched writing … [these] poor models that continually fortify the lawyer’s bad habits"