Why does legal English continue to remain archaic?

Perhaps this is a question for Law.SE if one exists, but I am asking here as there are other nice questions on English history.

There is some historical development account presented in Wikipedia, but I find it grossly inadequate to answer the question. English has always been a language that borrows and enriches itself, but retains its original flavour all the same. Why has a similar linguistic edification not happened with legal English? The question, in its elementary form:

• Why has legal English simply not moved on?

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.

As TimLymington and others have pointed out, the old, archaic words have strictly-defined meanings that a "more common" word does not have.

I'd like to also point out that legal documents must be unambiguous to an extent not required of most writing. Suppose in a novel the author writes, "Bob dropped by the office where his wife was waiting with her boss. He kissed her briefly and they left." Who did Bob kiss, his wife or her boss? Technically the language is ambiguous, but I think we would normally assume that the writer meant that he kissed his wife. If a few paragraphs later we learn that Bob is having an affair with his wife's boss and his wife doesn't mind, and so he is kissing her boss, we might be annoyed at the writer for the momentary confusion, but it's no big deal. We figure it out and move on.

But suppose a legal contract says, "Acme Trucking Company will deliver the shipment to the Beta Company warehouse. They will pay the invoice from Nadir Machine parts within 30 days." Who is the "they" who is supposed to pay for the shipment, Acme or Beta? Any reasonable person would say that of course the trucking company isn't expected to pay for the merchandise, that's the receipient's responsibility. But the text is ambiguous, and a lawyer could argue that the contract means that the trucking company must pay for the goods delivered to Beta. I'm sure a smart lawyer could come up with some plausible-sounding reason why this was part of the deal and thus the intended meaning of the contract.

I once got burned when a legal document that I was a party to mentioned a certain $4000 in assets, and then later in the document said that I had the rights to this $4000, but failed to clearly connect the reference that it was mine to the earlier statement describing it. So the lawyer on the other side said that this was a different $4000 and that the ownership of the first $4000 was therefore unspecified, and so his client was entitled to half of it. We eventually settled the matter by me agreeing to give him ... I forget how much ... toward the legal bills that his client was unable or unwilling to pay, and thus the client got nothing from the negotiations and it all went to the lawyer. But that's another story.

Anyway, my point is, legal documents are verbose because they dare not risk leaving any case not covered, because a smart lawyer for the other side may try to exploit any such ambiguity. While for most documents you could say, "Oh, just use your common sense", or "If you don't understand, ask me and I'll explain it," you can't do this with legal documents.

Could they be less verbose and use less mysterious language than they do? Probably. But it's not the slam-dunk easy thing to do that you might imagine.

Broadly speaking, I think the answer is "because there's not very much incentive for lawyers to change things" unless in a particular case particular wording is shown not to be adequate. The wordings/expressions that over time 'appear to work' stick, with little motivation to meddle with them.

Incidentally, this isn't a phenomenon unique to English.