Because if you study this stuff long enough, you'll find the LLMs hallucinate in this space far more than they do in a discourse or programming project.
It can be close enough to seem useful, but make critical errors that can cause rippling problems.
I've done a lot of the same sort of thing with Latin and chancery and while it can really help in understanding it on a personal level, it takes combing over what it has said and zooming in, comparing it with other information, and understanding the historical context. Otherwise it can completely mangle clauses, contexts, etc.
It is remarkable how well they can start to read things like chancery script, but again, many errors. Not reliable, especially in such large volumes. It would require extensive review. And they are probably getting flooded wth things like that now. And not all of them will be benevolent. History is laden with intentional corruptions and the flat, context-less reading that LLMs give can bury that even further.
Don't inundate researchers, churches, and records offices wth this stuff. But it is absolutely a space where it can help, with proper organization and engagement.