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).

The CEO (a lawyer) claimed that the lawyers who wrote these things would deliberately and unnecesarily overcomplicate them so that they could maximize billable hours.

I think they could quite easily have been templated using a DSL but that DSL would need frequent maintenance. These types of contracts did evolve as new types of clauses and legal constructs popped up and gradually evolved from "new" to "standard" to "boilerplate".

The problem than becomes that anyone who wants to understand the contract now has to read not just the contract as written, but also all of the definition of the DSL itself. This can actually be OK if the DSL is very commonly used, such as a DSL for contracts between two parties which sign new contracts every day.

But it is a huge waste of time for parties which rarely sign contacts, and is often used as an explicit moat to keep laymen from participating. If I give you a contract to sign that isn't even written in plain English, you will have no choice but to hire a lawyer specialized in understanding this contract DSL to advocate for you.

I imagine (savvy) lawyers actually love DSLs that purport to make contracts concise.

A DSL is an extra layer of abstraction above that. If you agree to a contract written in some DSL, then you must also agree to the way that DSL translates into plain English. To significantly compress a legal contract that is not deliberately written to obfuscate its meaning, the DSL has to pack a lot of precise meanings into every term, making it very dense and hard to parse unless you're well-versed in it.

The customers in our case did not actually look at the DSL - it was entirely internal. We decompiled the legal document into the DSL so that we could then represent the contract in more understandable ways.