I would have said the opposite. The fretting hand only needs to press onto the strings in the correct places whereas the picking hand is responsible for all the subtlety of expression in terms of how you attack the string.
Saying this as a professional mandolin player fwiw.
It's like Super Mario Bros. With a 4-way d-pad and 2 action buttons, why is the more complex 4-way on the left? Because most of the time you're holding a direction first with less requirement for precise timing, and then pressing the button at the correct instant depending on the movement. (We're talking general platformer play, not hyper precise d-pad moves for something like Super Metroid speedrunning.)
Even typing on a keyboard, I never hold a right-side shift or other modifier key, it's always left-shift then the target key.