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 also don't understand how one has trouble staying between the lines with minimal cognitive input after more than a few months of driving.
Once you have something assist you with that, you'll notice how much "effort" you are actually putting towards it.