Why not?
Take the physics engine in BOTW - the fire propagation and metal/weight physics are superb. RDR2 could benefit from those systems in a variety of ways. I'm not talking Magnesis, Cryonis and Stasis, but the realistic sandbox elements would offer more entertaining gameplay scenarios.
Wanna take out an enemy camp? Guns blazing approach, stealth or set the grass and trees on fire to either distract them, or kill them.
Because different games go for different goals. Putting a physics (and "chemistry") engine in an open world like Zelda will inevitably come at the expense of other aspects of the game. Zelda is a game that focuses on exploration, experimentation, and emergent narratives. For a more narrative-focused game like Red Dead, this doesn't make sense. Would it be cool if Red Dead had the extensive physics engine of Zelda? Sure, but I'm not sure it would benefit it that much. All of that stuff is tied into the design of the world itself. I don't think just taking the physics engine and putting it into other open world games necessarily makes them better. It's a different type of open world game