In a common law system, the benefit of the arcane language is that it's been tested in court before, so it has a definite meaning. In that sense, it's actually

In a common law system, the benefit of the arcane language is that it's been tested in court before, so it has a definite meaning. In that sense, it's actually "clear", even if it's pretty obscure to the layman. By contrast, novel wording has not been tested in previous cases, so no matter how clearly it might be written, there's a degree of uncertainty. A term doesn't have a definite legal meaning until a judge has made a ruling on it. That's why contracts and wills, particularly, often use weird phrasing.