Comment by tsimionescu

I believe that another thing that happens, and it is common in every field, is that certain constructions are very common within the field, and so the need arises among practitioners to shorten them. Most domains invent new words, but this doesn't work for laws and contracts (since they need to at least in principle be understandable to non-practitioners, such as most elected officials).

So instead of full-on jargon, legal texts get enshrined phrases, which practitioners can essentially skip over, but which also retain some meaning in plain English (though often sounding antiquated).