I'm assuming it's a pragmatic decision. Rather than creating a new class of thing ('rivers'? 'natural entities'?) and then revisiting the entire code of laws and updating it to apply to 'persons' and 'natural entities' where applicable, they made the pragmatic decision of leveraging the flexibility of the existing construct. A "legal person" is not a "human person" its a specific construct.

Think of it more as you have a top-level protocol in a piece of software. It's super flexible, designed for many diverse use cases. You write a whole pile of software around the top-level protocol "Person". Then, you realize you need to extend the software to support a new kind of thing, the "River". You now have a few choices.

Option 1: Massive refactor. Create a new top-level thing called "LegalEntity" to which both "Person" and "River" conform. Then, you have to update literally every single thing in the entire app to support both, and handle both appropriately.

Option 2: There's enough flexibility in "Person" than you can just make "River" conform to "Person" and you don't have to refactor anything at all.