It's not like they sat together in 1960 and decided to agree upon a set of latin terms for everyday occurrences - these are mostly inherited.
The idea is to keep the language reasonably stable that we can still confidently interpret legalese from 1812, and that in 2412 the legal body of 2012 can still be of some use without having historians fight over how we might have meant things.
This is particularly true in letter-of-law + case law jurisdictions - in spirit-of-law jurisdictions, interpretation is always required, while in non-case law jurisdictions, rulings don't have as much impact so you don't have to care about what was ruled 200 years ago.
More through inertia than desire, i.e. past performance is no guarantee of future returns. Change accelerates, and feudal times was not much more than 400 years ago in the scheme of things.
The idea is to keep the language reasonably stable that we can still confidently interpret legalese from 1812, and that in 2412 the legal body of 2012 can still be of some use without having historians fight over how we might have meant things.
This is particularly true in letter-of-law + case law jurisdictions - in spirit-of-law jurisdictions, interpretation is always required, while in non-case law jurisdictions, rulings don't have as much impact so you don't have to care about what was ruled 200 years ago.