Humans are simply incapable of paying attention to a task for long periods if it doesn't involve some kind of interactive feedback. You can't ask someone to watch paint dry while simultaneously expect them to have < 0.5sec reaction time to a sudden impulse three hours into the drying process.
Oh! And also, moving within the lane is sometimes important for getting a better look at what's up ahead or behind you or expressing car "body language" that allows others to know you're probably going to change lanes soon.
I commute mainly on the highway about 45-1hr each way every day and it makes a big difference for driver fatigue. I was honestly a bit surprised. Even though, I'm steering, it requires less effort. I don't have my foot on the gas and I'm not having to adjust my speed constantly.
Critically, though, I do have to pay attention to my surroundings. It's not taking so much out of my driving that I can't stay engaged to what's happening around me.