Roman law and old English law were actually used in real legal systems, and they are studied because they are the basis for the legal system we use today

I disagree with your second statement. I think they're studied because they are simple working legal systems which are easier to reason about than modern ones.

Arguably these legal systems are the conceptual basis for our modern law system because they are used pedagogically in this way.

In the US (and other countries) Roman and English Law are the "common law" and or "natural rights" that are referred to but generally not stipulated.

"New" governments had a habit of writing up founding documents that focus on the "new stuff" and waving in "all the old stuff" that doesn' conflict with the "new stuff" via a few throwaway phrasings.