Maybe its because of GTA V Online? GTA V Online for PS3 and Xbox 360 stopped getting content update in 2015.
I see what you mean, but i dont think RDR2 is too much different here. You also have tons of NPCs and horses/animals etc.. Also, items and collectibles that can be picked up. I'm not sure about destructable objects besides fences. These are also things that are in older games as well, but it doesnt necessarily mean that newer games arent more demanding because of that in this area. I'm no expert on this at all, but i think animation system for example requires its share of CPU power, and the animation system is much more complex than what we see in ARK (or other older games for that matter). How the AI routine is programmed can also be more complex. All of these things can add up.
It might be possible to get RDR2 to run on the Switch, but it might require tons of work, and it might not be feasable because of that. ARK also runs on phones by the way. I just downloaded it on my Samsung Galaxy S7 to test, and it works fine it seems. Framerate is maybe around 20fps if i should guess. Not the best, but at least its playable (i only tested for like 1 minute, but it runs). So if it can run on my relatively old phone, i wonder how demanding the game really is.