Exactly it's way harder to stay vigilant when nothing happens for hours. When that becomes days and weeks, it becomes near impossible.
I have a relatively "stupid" car by today's standards. It can only keep lanes, and adjust speed according to traffic and speed limits. Nothing more than a fancy cruise control really.
And even that makes mistakes, but it's simple enough that it doesn't create "situations" because you still have to actually drive yourself.

It's kind of funny when approaching a round about, it slows down automatically, and I enter the round about at a comfortable speed, but then suddenly half way through, it accelerates back to highway speed if you don't prevent it. This is extremely weird IMO, since you are still in the middle of turning to follow the round about?!
It took me a bit by surprise the first time that it suddenly accelerated in a situation where it absolutely shouldn't. But not much. because I was absolutely paying attention, as it can't navigate by itself.