The language of the law attempts to be consistent, legal English isnt entirely the same as ours, it remains somewhat inured to modernization. It is, if nothing a domain specific way of writing. (There is a great bit by Lenny Bruce on this very topic)
If a basic filing with some citations is needed, letting one AI generate it, and then another validate it (to catch soft ball hallucinations) leaves it to be read and reviewed.
Lawyers, good ones, are by trade, critical readers and reviewers of documents. The contempt exists for those who fail to do this part of the job. Letting AI write your submission without reviewing it, validating it, is telling.
I don't think that's quite it. Particularly in America's common law system, written laws are not "the law", in terms of what actually happens in enforcement, or in judgment when something ends up before a court. You have to deal with unwritten or disparately-documented standards, norms, case law, etc. LLMs don't know and can't account for curveballs that appear in practice; they'll correctly tell you what the law actually says, and work off that documentation, and blow your case, because that law isn't "the law" in its practical totality.
The lawyers with contempt for AI aren't necessarily the best "critical readers and reviewers of documents"; a good number just appreciate the substantial moat the current system keeps for them, and are contemptuous of 1) attempts to cross that moat without paying a toll, however likely that attempt is to fail, and 2) the way such failures highlight that our legal system doesn't work the way most people seem to think it does, because that increases the chances that the public will become dissatisfied and push for change.
I guess if it doesn't cross that moat, there cool with it?