It does run on steam deck (though at lower settings with FSR to get 40 mostly stable).
Open world games with some complexity generally take a decent amount of power. You have to load a good number of surrounding objects at any given time, with a pretty wide view on the zoomed out view. There are also other characters/animals doing stuff, environmental effects, and a healthy dose of passive checks on the environment against various traits of your party to see if your character identifies any of the secrets all over the world.