You’re actively engaged, Peter. Your butt muscles have to be continually active for you to stay in that chair.
Sitting isn’t passive. That’s why sitting for long hours is tiring. It uses butt muscles, thigh muscles and sometimes calf muscles.
It’s like glasses: They don’t just sit on your face passively. Your nose keeps them up. Your ears help.
Fall asleep and you’ll see just how passive it isn’t.