Because every attempt to change it makes the law more complex and more expensive. The archaic terms, like 'plaintiff' and 'writ' had clear definitions, partly because they had been hammered out over generations. A well-intentioned attempt to make it easier for the layman to understand (which every new Government tries to bring in) replacing them with 'claimant' and 'claim form' ends up causing confusion at the margins: if one partner in a marriage starts the divorce process but makes no financial claim while the other does, which one is the claimant? A writ was different in important respects from an originating summons; if both are replaced by a Claim Form, does the distinction survive? And, of course, confusion at the margins is where lawyers are needed, and where it gets expensive.
This is not pure guesswork on my part; in 2000, the British courts made a huge effort to simplify the civil courts' language and procedure. Everybody agreed that it was overdue, as the Rules of the Supreme Court were over a thousand pages long, and had to be re-issued every two years. Several of the brightest people in the country spent years rewriting the rules, and I was one of the people who had to try to make it work. It failed miserably; it turned out that words mean different things in different contexts (as any contributor here knows), so to replace one archaic term needed several near-synonyms, and each occurrence had to be scrutinised by the whole committee to see which applied. In practice, of course, this was impossible.
(As an analogy, try to debug a thousand-page program where each occurrence of each variable has to be replaced with one of six alternatives, but nobody knows which; and, of course, regular updates change both the variables and the structure while you're working. Large parts of it are safety-critical, since it bears on the liberty of the subject; and it has to be compatible with other systems across the world, in a variety of languages and with different philosophies.)
The current Civil Procedure Rule book is heading towards two thousand pages, and is replaced every year, with quarterly updates.
And, of course, the expense and confusion were not blamed on the politicians who ordered the change (and moved on before it happened), but on the lawyers and civil servants.
TL;DR: Because archaic English is clear and logical, while modern replacements are more confusing than helpful.
John Smith was born on April 1, which event was duly recorded.
To be even more legalesoteric
John Smith was born on April 1, the fact of which was duly recorded.
We are a bunch of pompous twits.
There is a move in some segments of the legal community (or the real community which has to deal with us lawyers) to return legal documents and dealings to human English. Simplify, simplify, simplify. And stop saying everything three times (albeit using synonyms). The sucess of this movement remains in doubt
Best Answer
This, like much legal language, is archaic. This particular case is a Non-Restrictive Relative Clause, which in ordinary English would just be
As Tim says, it's the extra folder that's the unnecessary padding, not the which. Which is utterly standard for non-restrictive relatives.
In this case, folder is Pied-Piped along with which to the front of the clause, in order to (try to) prevent any possibility of ambiguity about the referent of which. Repeating words or phrases redundantly, like the party of the first part (instead of he or they) is a standard practice for much legal language.
Pied-Piping is limited in modern English to prepositions with relative pronouns as objects, and NPs that command such prepositional phrases. Consider the sentence
This may be relativized as