Because as in software, so in law, you must deliver an end product that has the instructions expanded with the differences included in. You don't give templates, you dive the rendered result. A tech savvy law firm could of course use templates to define what that would be.

That said, a lot of state law is kind of like what you describe. An initial law, and tons of differences applied on it. E.g. article 3242 clarifies article 432 and adds the provision to do so and so.