I'm still convinced that it being called "full self driving" is misleading marketing and really needs to stop, since it isn't according to Tesla
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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.